Guitar Hero

To get this to work both of these conditions must be met

1: You will need to have unlocked the character previously on another difficulty level (such as normal)

2: You must NOT have played the desired level of play at all (so unlock them on medium to play with them on hard/expert, but do not play hard/expert at all)

To do the actuall glitch just go to medium and select either Izzy or Grim, and allow the game to save your progress, now head into hard or expert (assuming you have never touched these modes yet) and do NOT move the cursor when promted to select a hero, just hit green (or X) and select your guitar (when playing as Grim you'll notice that it works right here). When you start playing your hero won't be Axel Steel (or whoever was highlighted) but one of the two secret characters, don't ever change your character though because if you do you won't be able to play as them again until they are unlocked.