When you have completed the game the New Game+ feature will become available. In New Game+ mode you begin on the Legendary difficulty setting with your characters retaining the levels they had from the completed game.
When you enter the following codes quickly at the indicated screen the corresponding effect will become activated. You will not be able to save the game or earn Achievements when cheats have been activated.
Start Heroes (ALL Characters): During gameplay press Up(2), Down(2), Left(3).