Over the years, the order of trial has evolved to require that the prosecutor must prove their case completely (known as a prima facie case) before the Defendant needs to do anything. And even then, the Defendant never needs to prove anything in a criminal trial. However, what's important to remember is the way the human mind works, whoever gets to tell their story first has a distinct advantage over the other party. That's because when anyone hears a story for the first time, they allow it into their mind and it establishes itself as a new story. Then, when anything that's opposed to that first story arises (such as the Defendant's story or version of whatever happened) it is usually dismissed out of hand by the jury who heard the prosecutor's story first. Just think about your own experience in life. No matter how open minded you want to believe you are, once you've heard and internalized a story, anything that comes later that is inconsistent with the first story is denied out of hand.