How do you unlock all characters the fastest way?

Play in story mode. The further you get, the more charactors you unlock.
You can also get them from winning a lot of brawls. It will say "challenger approching" and some new charactor will play a 1 on 1 brawl with you. If you win, that charactor will be added to you charactor menu.
This is only on Super Smash Bros. Brawl.