By the end of the anime, Tsutomu and Birdy have been sharing bodies for so long that he'll have conversations with her, in front of other people, without even bothering to lower his voice or pretend he's talking on the phone. His friends pretty much think he's crazy.
The sequence in Decode's second season where Tsutomu is stuck in Birdy's body has to pretend to be Shion Arita. It begins with him being asked to pose in increasingly ridiculous costumes, up to and including a globe, and culminates in his being asked to eat a bowl full of crickets. When one of the crickets lands on his nose he screams!