Toward the end of the game, Mario and company are attempting to rescue Peach from Bowletta. Fortunately, Toadsworth had packed an extra dress for Peach in case of Clothing Damage. Mario's solution? DressLuigias Peach, then attempt to pass him off as the "real" princess. Luigi goes the extra mile; he mimics Peach's voice with surprising accuracy, and even covers his face with his hands to hide his mustache...until he is in Bowletta's cruiser.
Bowletta: By the way, Princess...why are you covering your mouth with your hands?
Fawful: Do you hide something?
Bowletta fakes Luigi out, and Luigi uncovers his face.
Bowletta: Fawful... Did you see that?
Fawful: Yes. Mustache.
Bowletta: ...Fire at will.
Hermie III thinking that Mario is Santa Claus because of his mustache and red clothes.
The whole sequence after Mario's consumed the Invincishroom. Luigi moving very slowly due to fear and being hypnotized into thinking that he's Mario in particular.
Mario running around his towel.
Practically every time Fawful is on screen.
Try flubbing the Bros. Attacks. The expressions are hilarious.