Jury instructions on an aiding and abetting theory of liability must be given when such derivative culpability forms a part of the prosecution’s theory of criminal liability and substantial evidence supports the theory. Appellant was convicted of robbery and kidnapping for purposes of robbery. Evidence was presented at trial that after drinking with appellant, the victim entered a vehicle driven by a woman. Appellant also entered the vehicle and as the woman drove a distance, he robbed the victim. On appeal, appellant contended that the trial court erred by failing to instruct sua sponte on accomplice liability. The Supreme Court agreed. As to the asportation element of the crime of kidnapping for robbery, there was substantial evidence that appellant acted as a direct perpetrator or as an accomplice to the crime. The jury could have determined that the driver’s conduct satisfied the asportation element as she locked the doors to the car and then drove a substantial distance during the robbery of the victim. Under this theory, appellant did not personally move the victim the requisite substantial distance required for asportation, but facilitated his accomplice’s conduct in moving the victim. At trial, the prosecutor relied in part on this theory. As a result, because accomplice liability for the kidnapping was raised by the evidence and necessary for the jury’s full understanding of the case, the court erred in not instructing on the theory. This error, however, did not constitute failure to instruct on an element of the offense and did not lessen the People’s burden of proof. The jury was fully instructed on all required elements, including asportation, as well as the People’s burden. Because there was no federal constitutional error, the court assessed prejudice under the People v. Watson (1956) 46 Cal.2d 818, 836 standard and determined that the error was harmless. There was compelling circumstantial evidence that appellant and the driver intentionally cooperated to kidnap and rob the victim and it is very unlikely that a properly instructed jury would believe appellant’s implausible claim that he did not intend to assist the driver in moving the victim.