The one that I was involved in, everyone played a part, and each person received a little booklet with information. The booklet had information on their character or part, including their motivation for doing the murder and what they knew about the events surrounding the murder. Someone played the Detective, and he went around throughout the night and "investigated" the murder, and people had to try to figure out who the killer was based on the information they had in their booklet and what they heard from other people.
The main rule was "You can not lie!" so that people wouldn't pretend to have different information than was in their booklets. We received three different booklets at different times in the night. After an hour or two we got the second one, and it gave us more information that we knew about the other characters and the situation, and then a little while later we got a third one with even more information.
The actual guilty person got their third one and the first thing on it said "You can lie now." No one guessed the right one, of course! Some of us ended up guessing ourselves because we thought that we had done it!
The one I went to was a costumed murder mystery, and everyone was dressed to the part, and it was set in the early 20th century so all the women we wearing dresses and the men were wearing suits with top hats. It was very fun. Good luck, and I hope you enjoy yourself!