After a lot of research and testing I've finally figured out how to get Luigi in Super Mario 64. It's pretty hard and VERY time consuming. Here's how:
1. Beat the game and get ALL 120 stars. Do not save, it will not work.
2. You must have ALL 120 STARS!
3. I repeat, ALL 120 STARS!
4. I cannot stress it enough, ALL 120 STARS!
5. YOU NEED ALL 120 STARS!
6. Okay, I'm done with the 120 stars.
7. Once you've got the stars, pause and push the reset button.
8. Turn the N64 back on and select the same file you used. It will seem erased but don't worry!
9. You will start back at the beginning. Go into the castle.
10. Luigi will talk to you! He will follow you around and you can switch places at will!
11. Enjoy!
There's a co-op mode too, but it's really poorly done. The camera is controlled by P1 and only follows P1. So you'll end up playing Luigi blind.
Goweegy!
April fools! Here's the real way to get Luigi:
1. Unplug your N64.
2. Sell it, SM64, and anything else necessary to get about $150.
3. Go to the store and buy a DS and SM64 DS.
4. Play SM64 DS and unlock Luigi.
5. Cry about your loss of an N64 and fight with guilty pangs.
6. Sell your DS for some money.
7. Buy your N64 and games back from the buyer at a hefty upcharge.
8. Be happy with just Mario and swear that you'll never try to get Luigi again!
Happy April 1st!