http://en.wikipedia.org/wiki/Accessory_%28legal_term%29
An accessory is a person who assists in the commission of a crime, but does not actually participate in the commission of the crime as a joint principal. The distinction between an accessory and a principal is a question of fact and degree:
The principal is the one whose acts or omissions, accompanied by the relevant mens rea, are the most immediate cause of the actus reus (Latin for "guilty act"). If two or more people are directly responsible for the actus reus, they can be charged as joint principals (see common purpose. The test to distinguish a joint principal from an accessory is whether the defendant independently contributed to causing the actus reus rather than merely giving generalised and/or limited help and encouragement.
