What is a jury's duty to instruct on general principles of law in a criminal case?

In criminal cases, "'"even in the absence of a request, the trial court must instruct on the general principles of law relevant to the issues raised by the evidence. [Citations.] The general principles of law governing the case are those principles closely and openly connected with the facts before the court, and which are necessary for the jury's understanding of the case."'" (People v. Breverman (1998) 19 Cal.4th 142, 154.) This duty "'encompasses an obligation to instruct on defenses . . .'" (People v. Lopez (1992) 11 Cal.App.4th 1115, 1120) that are "supported by substantial evidence [and] that are not inconsistent with the defendant's theory of the case." (People v. Montoya (1994) 7 Cal.4th 1027, 1047).

"[T]he existence of 'any evidence, no matter how weak' will not justify instructions . . . , but such instructions are required whenever evidence . . . is 'substantial enough to merit consideration' by the jury." (People v. Breverman, supra, 19 Cal.4th at p. 162.) "'Substantial evidence' . . . is "'evidence from which a jury composed of reasonable [persons] could . . . conclude[ ]'" that the [defense] was [applicable]." (Ibid.) In making this assessment, the court is not to assess the credibility of witnesses, a task for the jury. (Ibid.)