Distinguish between the terms actus reus and mens rea. How are they significant in criminal law?

To what standard of law must the defendant’s mens rea be proven in order to gain a criminal conviction? Must the state prove “what the defendant was thinking at the time of the crime” in order to prove mens rea? Why or why not?

To what standard of law must each element of the actus reus be proven, and why?

Which of the two legal requirements listed above (i.e., actus reus and mens rea) is more difficult to prove beyond a reasonable doubt in a trial, and why?