What is the test for whether a defendant has committed an affirmative act or a crime of omission?

An accused must do a guilty act (actus reus), or omit to do a required act, to be guilty of a crime. A statutory expression in California of this fundamental concept is in Penal Code section 20, which provides: "In every crime or public offense there must exist a union, or joint operation of act and intent, or criminal negligence." (See also Pen.Code, 15 [definition of crime includes act, law forbidding or commanding it, and prescribed punishment]; People v. Crutcher (1968) 262 Cal.App.2d 750, 754, 68 Cal.Rptr. 904 [some act, committed or omitted in violation of law forbidding or commanding it, is necessary for there to be crime].) We are dealing here with a crime requiring an affirmative act (bringing drugs into a jail), not a crime of omission (e.g., failing to file a tax return). As a result, the first question we must answer is whether defendant did an affirmative act.