What constitutes a principal in the commission of a crime?

"Principals in the commission of a crime include both the direct perpetrator of the crime and those who aid and abet the commission of the crime. [Citations.] A person aids and abets commission of a crime when, with knowledge of the perpetrator's unlawful purpose, he, by act or advice, encourages or facilitates commission of the crime with the specific intent to do so." ( People v. Mejia (2012) 211 Cal.App.4th 586, 606, 149 Cal.Rptr.3d 815.) However, "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, 1117, 108 Cal.Rptr.2d 188, 24 P.3d 1210, some italics added.)