Allsup appeals from his conviction for two bank robberies (18 U.S.C. § 2113(a)), contending that multiple errors in the course of his trial denied him a fair trial. We agree with him that the cumulative errors mean that he must be tried anew.
The two bank robberies occurred in Tucson, Arizona, in October, 1976, two weeks apart. The Valley National Bank was first robbed and the First National Bank was robbed thereafter. Two factual issues dominated the proceedings: (1) Was Allsup the person who committed either or both of the robberies, and (2) did Allsup have the mental capacity to have committed either or both of the robberies? The prosecution’s evidence on both issues was not so insubstantial as to render the conviction vulnerable to a challenge on appeal based upon the insufficiency of the evidence, but neither was the evidence so strong that one or more errors could not have affected the outcome.
The Government presented only one identification witness in respect of the first robbery, and she testified that she was “not sure” that Allsup was the robber. The district judge acknowledged that the witness could not identify Allsup as the man who robbed the bank. Identification thus depended upon the jury’s evaluation of the testimony of a Government fingerprint expert who testified that a fragment of a fingerprint found on the note that the robber gave to the teller belonged to Allsup.
Evidence that Allsup robbed the First National Bank was stronger because he was carrying a bill that was part of the First National’s bait money when he was arrested. However, the identification testimony was conflicting. Two of the witnesses to the second robbery selected from an FBI *70photographic spread photograph 1 as the robber’s picture and also identified Allsup in the courtroom as the person depicted in photograph 1. In fact, Allsup was not the man who appeared in photograph 1; his picture was photograph 4.
Allsup introduced substantial testimony to cast doubt upon his sanity. Although we think that the Government adequately rebutted the defense, the sanity issue was nevertheless a close one for the jury to resolve.
Of the many contentions of error, we find it necessary to address only four: (1) The inadequacy of voir dire of the jury on the insanity question; (2) the failure of the district court to excuse two jurors for cause; (3) the district court’s intervention in cross-examination of a Government witness which resulted in an inflammatory remark from the witness; and (4) the correctness of the district court’s refusal to grant Allsup’s motion to sever the two bank robbery counts. The other contentions are not sufficiently meritorious to require comment.
I
Allsup’s counsel requested the court to ask members of the panel on voir dire whether any member of the panel “feels he could not clearly sit on this case because evidence of insanity or lack of mental capacity might be introduced into evidence.” During the court’s conduct of voir dire examination, the following dialogue occurred:
“THE COURT: Anybody else? Thank you. One or two more questions. Is there anyone on this panel who feels that he or she cannot sit on this case because evidence of insanity or lack of mental capacity — I’m afraid of the next word. If the word is going to be pressed I won’t ask the question.
“No answer? I won’t ask the question.
“MR. KAY (defense counsel): I would ask for the question to be asked, Your Honor.
“THE COURT: You ask me to ask the question and I refuse to ask it unless part of that verb be either . omitted or changed.
“MR. KAY: I’m not prepared to do that at this time.
“THE COURT: Very well. You have an exception.
“Is there anyone who feels that they cannot do that at this time for any reason? The answer is no.”
Allsup had the right to have prospective jurors questioned about their ability objectively and fairly to evaluate the evidence of insanity or lack of mental capacity. (Cf. United States v. Jackson (7th Cir. 1976) 542 F.2d 403, 413; Brundage v. United States (10th Cir. 1966) 365 F.2d 616, 618.) The Government does not quarrel with that proposition; rather, the Government argues that the district court, in substance, brought the insanity defense to the jury’s attention, even though the court did not ask the defendant’s question in the form in which it was originally requested. We do not think that a fair reading of the quoted dialogue supports the Government’s claim. The district court did not refer to the insanity defense in any way on voir dire except for the quoted statements. The court’s statements, in context, do not convey to us, and we do not think they could have conveyed to the jury that the court was asking the jurors to examine their attitudes about a potential insanity defense. On the contrary, the substance of the court’s statements was a refusal to ask the jurors about an insanity defense.
The district court’s failure to question the jurors about their attitudes toward an insanity defense cannot be justified by the refusal of defense counsel to promise that he would introduce evidence of insanity. Defense counsel could not be expected to predict with certainty the course of the trial. For example, after the Government’s case in chief, defense counsel might have decided to rely on weaknesses in the Government’s case without putting on any evidence for the defense. The district court cannot require a commitment to produce evidence relevant to an insanity defense as a condition precedent to interrogating jur*71ors on voir dire about their reactions to a potential defense based on lack of mental capacity or insanity. (Cf. United States v. Robinson (1973) 154 U.S.App.D.C. 265, 475 F.2d 376, 380 (“If the issue is one on which jury attitudes should be probed on voir dire to assure a fair trial by an impartial jury, this strength of our system should not be scuttled merely because, on relatively infrequent occasions, a planned defense is unexpectedly foreclosed or abandoned.”).)
II
Two of the prospective jurors during voir dire examination disclosed that they worked for the First National Bank, one of the banks that Allsup was charged with robbing. Neither of the prospective jurors worked at the branch that had been robbed. Both jurors responded to an additional inquiry by the court by stating that they would be able to decide the case fairly despite their employment. Counsel for Allsup moved to excuse the two bank employees for cause. The court denied the motion. Thereafter, Allsup’s counsel used two of his peremptory challenges to excuse those. two jurors. Allsup’s counsel ultimately exhausted all of his peremptory challenges. We agree with Allsup that the court erred in denying the motion to excuse the two bank employees for cause, thereby compelling him to use up his peremptories.
“[T]he right to a jury trial guarantees to the criminally accused a fair trial by a panel of impartial, ‘indifferent’ jurors.” (Irvin v. Dowd (1961) 366 U.S. 717, 722, 81 S.Ct. 1639, 1642, 6 L.Ed.2d 751.) The princi pal way in which this right to trial by “indifferent” jurors is secured is through the system of challenges exercised during voir dire. Inhibition of the right to challenge peremptorily or for cause is usually deemed prejudicial error, without a showing of actual prejudice. (See, e. g., Swain v. Alabama (1965) 380 U.S. 202, 219, 85 S.Ct. 824, 13 L.Ed.2d 759; United States v. Turner (9th Cir. 1977) 558 F.2d 535; United States v. Nell (5th Cir. 1976) 526 F.2d 1223, 1229; United States v. Boyd (5th Cir. 1971) 446 F.2d 1267, 1275, n. 27.) “[I]t is error for a court to force a party to exhaust his peremptory challenges on persons who should be excused for cause, for this has the effect of abridging the right to exercise peremptory challenges.” (United States v. Nell, supra, 526 F.2d at 1229.)
A court must excuse a prospective juror if actual bias is discovered during voir dire. Bias can be revealed by a juror’s express admission of that fact, but, more frequently, jurors are reluctant to admit actual bias, and the reality of their biased attitudes must be revealed by circumstantial evidence. We agree with the observation in Kiernan v. Van Schaik (3d Cir. 1965) 347 F.2d 775, 781: “That men will be prone to favor that side of a cause with which they identify themselves either economically, socially, or emotionally is a fundamental fact of human character.” The potential for substantial emotional involvement, adversely affecting impartiality, is evident when the prospective jurors work for the bank that has been robbed. Persons who work in banks have good reason to fear bank robbery because violence, or the threat of violence, is a frequent concomitant of the offense, (See, e. g., United States v. Poole (3d Cir. 1971) 450 F.2d 1084 (Claimed error in denying challenge for cause of jurors who were, or who were related to, bank tellers in prosecution for bank robbery was a “contention of some significance.”); Sims v. United States, (1968) 132 U.S.App.D.C. 111, 405 F.2d 1381, 1384, n. 5 (Excuse for cause of potential jurors who are cab drivers or who are related to cab drivers in a case arising out of the fatal shooting of a cab driver during a robbery attempt.).)1 The employment rela*72tionship coupled with a reasonable apprehension of violence by bank robbers leads us to believe that bias of those who work for the bank robbed should be presumed.
The prospective jurors who worked for the bank robbed should have been excused for cause. Excusing those prospective jurors would have caused a slight inconvenience, which was heavily counterbalanced by the substantial probability that these prospective jurors, despite their disclaimers, could not become the “indifferent” jurors which the Constitution guarantees a criminal defendant.
Ill
June Bentley, a First National Bank teller, was an identification witness for the Government. Outside the presence of the jury, Ms. Bentley was shown a series of photographs that had previously been admitted into evidence. She identified photograph 1 as a picture of Allsup and as the person who robbed the bank. On redirect examination, the Government elicited testimony from her that she had meant that the robber was actually the person shown in photograph 4. Photograph 4 was a picture of Allsup. The Government prosecutor sought an explanation for the witness’ change in her testimony, and she replied, “Well, I’ll tell you, as I look at them right now — I’m scared. As I look at it right now, it is number 4.” In the presence of the jury, Allsup’s counsel brought out on cross-examination the fact that the witness had changed her testimony moments before and that she had previously identified photograph 1 as the picture of the robber. The trial court intervened, and the following colloquy occurred:
“THE COURT: And did you say why you made the mistake?
“THE WITNESS: Yes, I did.
“THE COURT: What did you tell us then?
“THE WITNESS: Well, when I first glanced at them — I don’t know, I’m scared and my mind just went blank.
“THE COURT: Are you talking about how you feel today?
“THE WITNESS: Yes, sir.
“THE COURT: All right. Then you made a mistake?
“THE WITNESS: Yes, sir.”
The district court, of course, is much more than a “moderator or umpire.” (Smith v. United States (9th Cir. 1962) 305 F.2d 197, 205.) The district court may properly participate in the examination of witnesses for the purpose of “clarifying the evidence, controlling the orderly presentation of the evidence, confining counsel to evidentiary rulings, and preventing undue repetition of testimony.” (United States v. Malcolm (9th Cir. 1973) 475 F.2d 420, 427.) But the court must also be mindful that in the eyes of a jury, the court occupies a position of “preeminence and special persuasiveness” (United States v. Trapnell (9th Cir. 1975) 512 F.2d 10, 12), and, accordingly, the court must avoid the appearance of giving aid to one party or another. (See, e. g., United States, v. Pena-Garcia (9th Cir. 1974) 505 F.2d 964, 967.)
The district court’s intervention in this situation rehabilitated a prosecution witness whose credibility had been seriously undermined by defense counsel. (See United States v. Nazzaro (2d Cir. 1973) 472 F.2d 302, 307.) The district court knew from the testimony that had been taken out of the presence of the jury that the witness was going to state that she was “scared” at the time of the trial. The implication was strong that Allsup had frightened the witness, not simply that she was suffering from a common case of nervousness from *73being in a courtroom. The impact of the district court’s interrogation was to destroy the effect of telling cross-examination, to rehabilitate the witness, and to give her testimony an extra and a potentially prejudicial persuasiveness. (Cf. United States v. Moorman (7th Cir. 1966) 358 F.2d 31.)
IV
No abuse of discretion was shown in the refusal of the district court to grant Allsup’s motion to sever the two bank robberies for the purposes of trial. We are aware that the Government’s case on both counts was strengthened by trying the two bank robberies together. However, the two robberies were carried out in a similar way and occurred within two weeks of one another. The district court, pursuant to Rule 8 of the Federal Rules of Criminal Procedure, did not abuse his discretion in refusing to sever the two bank robbery counts. (United States v. Ragghianti (9th Cir. 1975) 527 F.2d 586.)
REVERSED AND REMANDED FOR A NEW TRIAL.
. The potential for bias on the part of a prospective juror who is employed by the bank, a branch of which is alleged to have been robbed by the defendant, is very great. On the other hand, potential bias is extremely attenuated, or non-existent, when the only connection between the victim and the prospective juror is that of employer and employee in a setting in which the employee is working in an area substantially removed from a threat of the crime charged, and, particularly when the crime itself does not involve any apprehension of violence. *72Thus, for example, in Frazier v. United States (1948) 335 U.S. 497, 69 S.Ct. 201, 93 L.Ed. 187, no abuse of discretion occurred in refusing to dismiss for cause in a narcotics case prospective jurors who were employees of the Treasury Department, when the jurors were not connected with the narcotics bureau within the department. Similarly, in United States v. Boyd, supra, 446 F.2d 1267, no abuse of discretion was involved in the district court’s refusal to strike for cause employees of an Air Force base in a prosecution for receiving tools stolen from the base, where there was no showing that the jurors were involved in base security or were employed in the department from which the tools were stolen.