The former is primarily concerned with the 'physical' aspect of criminal responsibility whereas the latter is concerned with the 'mental' aspect.
In understanding actus reus one must establish from the very beginning that it constitutes something much wider than just a criminal act. In some instances failure to act or omission constitute actus reus of an offence. It is usual that just a physical act is required to satisfy actus reus requirements. However, in certain circumstances must prevail and certain consequences must follow from the action/s of the accused.
In relation to statutory offences, such statutory provisions and decided cases that have interpreted them will indicate what constitutes the actus reus of a particular offence but may not necessarily indicate that whether or not liability can arise by means of an omission.
Accordingly a useful working definition of actus reus is that it comprises all the elements of the definition of the offence except those that relate to the mental element (mens rea) which is required on the part of the accused. (Allen, 1991, 18).
In understanding the term mens rea it is not easy to decide on the evidentiary criteria for something as hard to define as mental state of a given offender. They are physical control at the time the act took place. ...