Star Coin Guide/Walkthrough/FAQ
While selecting a level in Single Player mode, hold ZL to play as Nabbit. Note: Luigi will return as the main character after completing the level or if Nabbit dies.
When selecting the final Star Road level in Single Player mode, hold ZL.
Successfully complete the game by defeating the final Boss. Then, hit the red "M" block that appears at the start of each level to make Luigi move like Mario in New Super Mario Bros. U.