Physical element of the crime.
It can be an act, or a failure to act (an omission) or a state of affairs.
The actus reus must have an act or an omission that results in a consequence.
Voluntary nature of actus reus:
An act or an omission must be voluntary on the part of the defendant.
If the defendant has no control over his actions then he has not committed actus reus.
Hill v Baxter (1958) where the Court stated that the driver of a vehicle could not be said to be doing the act of driving involuntarily if, for example, he had a heart attack while driving or was hit on the head with a stone.
Involuntary acts and assaults:
e.g. defendant hits another person dye to reflex action or a muscle spasm.
State of affairs
Very rare, state of affairs is offences like getting caught in possession of drugs.
The general rule for omissions is that there is no liability for an omission in criminal law unless there is a duty of act.