If you want to put on a murder mystery show, you must start with a script. It's a breeze if you follow these simple steps.
A murder mystery script is one part script and three parts improv. The basic ingredients are: someone must die. Somebody else must kill that person. Multiple people must have motives. And somebody must lead the investigation into the murder and guide the audience to a decision.
Create a cast list: One victim, 3 to 5 potential murderers and one Investigator. Give each of the potentials substantial motive. Affairs, Bad Business Deals, Inheritance, Rivalry are just a few. Each potential must have a history with the deceased. It's best if the investigator is an innocent (an off duty officer) or is secretly investigating the deceased.
Write out a list of things that must be communicated. Each character must say specific lines that communicate their motive and these lines must be heard by the bulk of the audience. (For example the deceased's wife might say "I've seen you with that woman." Or his business rival might say "Say that to me again and I'll kill you.")
Write out an order of events. It works well if the deceased is the host. That way he can clue the audience in on the nature of the night. Have each character make an entrance (except for the investigator). Have each potential murderer have a vocal or physical conflict with the deceased. At the end of the meal, have the host stand up to give a toast. At this point he either drinks poison or the lights go out and when they come back on he is dead. This is when the investigator jumps up and takes over the evening. Have a couple of the characters drag the deceased out of the room (so the actor doesn't have to lay there all night). Then let the fun begin. Have the investigator ask questions of the audience. Any information that needs to be revealed that the audience doesn't bring up can be brought up by the cast members. The investigation should last no more than half an hour. At the end, after the murderer is discovered and taken away, have the entire cast return and say thanks to the audience.
Aim for a stylized setting; Prohibition, Old West, Modern Mafia Set the script up so that any one of the potentials could be the murderer that evening.
Always have an after death escape route. Warn anyone present who isn't part of the mystery dinner that the death is fake (no 911 calls).