Legal Question in Criminal Law in Pennsylvania

Seriously? Is this a homework assignment?

In any criminal case the prosecution has to prove that (a) that a crime was committed; and (b) that the defendant was the one who committed it. The defense has to prove either that (a) no crime occurred or (b) that the defendant did not do it (alibi) or else (c) that the defendant had an excuse (like insanity or self-defense/justification).

Lawyers cannot testify as to what happened. Only witnesses can do that. The way to get witnesses to testify is for the prosecution and defense to ask questions. If the witness will testify favorably to the prosecution, the prosecution calls the witness. The defense gets to cross-examine the witness also by asking questions. The purpose of cross-examination is to test whether the witness is telling the truth and/or whether the witness remembers the details correctly. If the witness will testify for the defense then the defense attorney asks the initial questions and the prosecutor gets to cross-examine the witness.

The movie "My Cousin Vinny" is a great movie involving the examination and cross-examination of witnesses and shows why it is important. In the movie the prosecution's case slowly falls apart by the cross-examination of witnesses and the defense calling its own witness. The defense attorney shows through the course of the movie why the defendants did not commit the crime and how it was committed by someone else in a case of mistaken identity.