Indirect Forms of Crime
A criminal conspiracy is an agreement between two or more people to achieve something unlawful. The actus reus of a conspiracy is formed in the agreement, either to commit a crime or to achieve an outcome which though not unlawful, is still dishonest or tortious, by collaborative means. The latter part of the definition exists to capture the guilt of persons acting together to achieve something that would not be a crime if done by just one person. For example, in Howes (1971) conspirators came together to falsify school examination results and were found to be guilty of a conspiracy.
A conspiracy can take the form not only of words, but also of gesture signalling assent to the crime. It can also take the form of overt actions in furtherance of the crime, which can be seen to signify that an agreement between them had taken place.
Conspiracies can be structured various ways. Notably, there are “wheel” conspiracies in which a central figure, takes advice from a number of different conspirators, and there are chain conspiracies in which a number of conspirators discuss the crime in turn, one after another with no central figure.
The mens rea of a conspiracy is twofold. First, there is the intention to affect the agreement and secondly, the conspirator(s) must realise the plan is unlawful. This is to be judged on objective grounds - that is, the defendants' knowledge is to be assumed that of a reasonable person in the position of the defendant. As for the actual opportunity to offend rather than merely to propose an act, it is considered impossible for a person to have conspired in the relevant sense to do something that is impossible for them.
A conspiracy is to be distinguished from an accessorial offence in that the conspirators are all principals. If any merely encourage, procure or otherwise support from the sidelines, then they are not conspirators but accessories.
 R v Howes  2 SASR 293.