Leftover in Sonic Superstars is a debug menu allowing you to use a level select and sound test. It’s very simple to get to this screen.
Sonic Superstars How to Access the Debug Menu
Your only requirement is to be on a save file that has beaten the game (100% not required).
While on the main menu, relative to an Xbox controller press the following button combination:
X, X, Y, Y, X, X, X, X. And you should see this screen.
This makes it easy to get around levels with different characters without having back out through slow menus. Unfortunately you cannot make characters go into other character’s exclusive acts.