A murderer for the mystery is selected randomly at the beginning of the game.

"When play begins" is the best point to initialize any aspects of the game that are meant to change between playings. For instance, in this scenario, we would randomly select one of the other characters to be guilty of murder:

"Clueless"
The murderer is a person that varies.
When play begins:
   now the murderer is a random person who is not the player.
The Billiards Room is a room. Colonel Mustard and Professor Plum are men in the Billiards Room. Miss Scarlet and Mrs White are women in the Billiards Room.
Instead of examining the murderer:
   say "[The noun] certainly looks fiendish!"
Test me with "x mustard / x plum / x scarlet / x white".
Test me with "x mustard / x plum / x scarlet / x white".
Billiards Room
You can see Colonel Mustard, Professor Plum, Miss Scarlet and Mrs White here.

>(Testing.)

>[1] x mustard
Colonel Mustard certainly looks fiendish!

>[2] x plum
You see nothing special about Professor Plum.

>[3] x scarlet
You see nothing special about Miss Scarlet.

>[4] x white
You see nothing special about Mrs White.