In State v. Fisher, 141 Ariz. 227, 251, 686 P.2d 750, 774 (1984) the Arizona Supreme Court noted motions for a new trial based on newly discovered evidence are "disfavored" and "should be granted with great caution." The court further observed that a trial court, in evaluating the effect of new witness testimony, properly may consider the credibility of that witness without "usurping the function of the jury." Id. And, the court in Fisher considered evidence not presented to the jury, including a letter the witness had written to the defendant after he had been found guilty. Id.