If the prosecution can not prove that the defendant acted voluntarily, then their case would fail. There are of course, in some circumstances, where the defendant may perform an involuntary act, such as when the accused is not in control of their own body. In this type of case, it can be said that the accused is acting in a state of automatism. Seago (1994) suggests that on the face of it, it seems to be an easy option for the defendant to swear that they blacked out or were not in control of their own body at the time the offence took place. It would also be very difficult for the prosecution to prove that they had not. But when put into practice, it is perhaps not as easy as one would first assume, as there are limitations to using automatism as a defence. The case of Broome v. Perkins (1987) helps to demonstrate this point. In this case, the defendant was charged with driving without due care and attention, and was found to have driven erratically during his five-mile journey. But the defendant claimed that he hadn't recalled anything of the journey. In this case, medical evidence suggests that it is perfectly possible for a person in his condition to react to significant stimuli. Therefore, the divisional court held that this did not amount to automatism, as it is only when someone reaches a state of complete lack of control that the defence of automatism exists. Lord Denning stated that automatism is confined to "acts done while unconscious and to spasms, convulsions and reflex actions.".
There is of course, in some situations, that the accused may have failed to act, and this then led to the prohibited event occurring. This allows the actus reus to cover omissions, as well as an act. There are some situations where the accused has a duty to act, and could therefore be held liable for a true omission. This can be demonstrated in the case of R v Gibbons and Proctor (1918,) where a man and a woman were living together with the man's daughter.