What is the duty of a court to instruct a defendant on a particular defense?

A trial court has a duty to instruct on a particular defense "'only if it appears that the defendant is relying on such a defense, or if there is substantial evidence supportive of such a defense and the defense is not inconsistent with the defendant's theory of the case.'" (People v. Barton (1995) 12 Cal.4th 186, 195.) "Although a trial court should not measure the substantiality of the evidence by undertaking to weigh the credibility of the witnesses, the court need not give the requested instruction where the supporting evidence is minimal and insubstantial." (People v. Barnett (1998) 17 Cal.4th 1044, 1145, fn. omitted.)