The elements that constitue mens rea, namely: 1. The accused knew what they were doing 2. They knew what they were doing was wrong (legally) 3. They were in sound mind to choose whether or not to do it 4. They chose to do it anyway. If any of these 4 are not present, then mens rea is not complete, and the person can be found not guilty (including pleading insanity).
Furthermore, there is a category of offences known as “strict liability offences”, for which there need not be any evidence of mens rea i.e. you can be found guilty of commiting a crime even without knowing it. This includes offences such as dog fouling, breaking the highway code etc. Essentially, it says that ignorance of the law is no defence, you’re guilty through the actus reus alone.
On the other hand, often the opposite is true, mens rea in itself can lead to conviction i.e. if it was proved that a person was planning/intending to commit a crime, they can be convicted without actually having “done” anything. Usually, this takes the form of fulfilling an offence in itself e.g conspircay to commit murder. This has a great deal of importance today; if someone purchases a load of chemicals and mixes them to create explosives in their home, the law doesn’t have to wait for tem to blow something up before they can be convicted of terrorism. If you’ve seen the film “Minority Report”, you’ll know the dangers of taking this to the extreme.
Broadly speaking, in criminal law, it must be proved beyond reasonable doubt that a person pposessed the relevant mens rea and committed the actus reus on order to be convicted of a crime. However, to cope with the variety of offences, to encourage awareness of the law and to ensure that the law has a moral element in its operation, then the two doctrine can stand on their own to lead to a conviction.