What is the legal test for a person who aids and abets a crime?

We begin with an exposition of relevant legal principles. "'All persons concerned in the commission of a crime, . . . whether they directly commit the act constituting the offense, or aid and abet in its commission, . . . . are principals in any crime so committed.' [Citations.] Thus, a person who aids and abets a crime is guilty of that crime even if someone else committed some or all of the criminal acts. [Citation.] Because aiders and abettors may be criminally liable for acts not their own, cases have described their liability as 'vicarious.' [Citation.] This description is accurate as far as it goes. But . . . the aider and abettor's guilt for the intended crime is not entirely vicarious. Rather, that guilt is based on a combination of the direct perpetrator's acts and the aider and abettor's own acts and own mental state." (People v. McCoy (2001) 25 Cal.4th 1111, 1116-1117.)