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actus reus = the fact that a criminal act has been committed
mens rea = the individual accused was, at the time of the crime, in a mental state to understand that what he/she was doing was wrong / unlawful and thus that there was intent present in the commission of the crime.

In order to be prosecuted for a crime, in most Western countries, the prosecution has to prove that both actus reus and mens rea are present.

Even a defendant displaying symptoms of a mental disorder at time of trial will have to prove that the disorder was a) present at the time they committed the crime and b) the crime was a direct result of the mental illness (although the strict McNaughten rules are no longer applied in all states / countries in the US / UK, under current mental health legislation similar laws are in place - even though debate on the exact nature of what they should include is ongoing and different rules are being applied across states / countries).

Even when both have been established, the competence to stand trial might still be at issue, depending on the current mental state of the defendant (i.e. can s/he instruct their lawyer, understand proceedings in court etc), but unless actus reus and mens rea are present, a court case will not go ahead.
exceptions are strict liability cases such as accidentally running over a person, in which case mitigating circumstances with regards to intent / resposibility can be used as a defense.

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