Go to the last available character in multiplayer (usually Mishkin) and hold
1.Hold L and R and press C-left
2.Hold the L button and press C-up
3.Hold the L and R buttons and press left on the pad (not the stick thingy).
4.Hold L and press right on the pad.
5.Hold R and press down on the pad.
6.Hold L and R and press C-left.
7.Hold L and press C-up.
8.Hold L and R and press right on the pad.
9.Hold L and R and press C-down.
10.Hold L and press down on the pad.
WHEW! there is no sound to verify it worked but you should see almost 40 extra characters like the enemy heads, a grandma and even a terrorist complete with face mask.
this trick however is not saved so you will have to repeat it when you play again.
This trick is going to work I promise, if it doesn't you probably have some defect or you moved the anolog stick in the process.