To get all characters, just simply beat the game on singles career with any wrestler on any difficulty. After a Pay-Per-View event has taken place or you've just won a belt, it will say something has been unlocked, please save the Utillites. The codes are in the cheat menu on the main menu. NOTE: If you beat Singles than beat Tag Team, you don't get anything else.

The wrestlers and extras you get are:

Shawn Michaels
Chyna
Sable
Marc Mero
Head
Sgt. Slaughter
Kurrgan
Taka
Jacqueline
Jerry Lawler
Trainer
Big Head Mode
Ego Mode
Extra Attributes for a created wrestler
New clothes for a created wrestler