For participatory liability there must be a joint enterprise. The mens rea for secondary participation is the intention to do the act which aids or abets. A principal offender must also have the necessary mens rea to be guilty of the offence. Where two or more people do the actus reus with the required mens rea then they are all principals. A secondary party can be convicted even though the principal is acquitted, if the actus reus was committed, but the principal lacked the required mens rea or has a defence not available to the secondary party. If the principal has attempted the main crime then the secondary party can be guilty as an accessory to the attempt. Where spontaneous violence has occurred then it is possible for the secondary party to withdraw effectively by walking away. Acts done after the crime has been committed are not done as a secondary party but may be a separate substantive offence.