What is the legal test for an aider and abettor in a murder case?

At trial, the prosecutor argued that Boston was guilty of murder under an aiding and abetting theory. "[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." (People v. McCoy

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(2001) 25 Cal.4th 1111, 1117.) "[A] person aids and abets the commission of a crime when he or she, acting with (1) knowledge of the unlawful purpose of the perpetrator; and (2) the intent or purpose of committing, encouraging, or facilitating the commission of the offense, (3) by act or advice aids, promotes, encourages or instigates, the commission of the crime." (People v. Beeman (1984) 35 Cal.3d 547, 561.) "Thus, proof of aider and abettor liability requires proof in three distinct areas: (a) the direct perpetrator's actus reusa crime committed by the direct perpetrator, (b) the aider and abettor's mens reaknowledge of the direct perpetrator's unlawful intent and an intent to assist in achieving those unlawful ends, and (c) the aider and abettor's actus reusconduct by the aider and abettor that in fact assists the achievement of the crime." (People v. Perez (2005) 35 Cal.4th 1219, 1225.) "[N]either presence at the scene of a crime nor knowledge of, but failure to prevent it, is sufficient to establish aiding and abetting its commission. [Citations.] However, '[a]mong the factors which may be considered in making the determination of aiding and abetting are: presence at the scene of the crime, companionship, and conduct before and after the offense.'" (People v. Campbell (1994) 25 Cal.App.4th 402, 409.)