The actor has played the wealthy inventor-turned superhero in two Iron Man movies and reprises the role for new blockbuster The Avengers.
Downey, Jr.'s onscreen characters often affect his personality while he is working on a movie project, and his partner Susan was horrified to see him displaying the ""cocky"" characteristics of womanising party boy Stark at home.
However, Susan admits she found the personality shift strangely sexy.
She tells Entertainment Tonight, ""It's my least favourite character he takes on... He is a little cockier, but it's very sexy... Well Tony's really sexy...""
Downey, Jr. adds, ""When I'm playing Sherlock, she'll tell me, 'I'd ask you a question but I know you already have the answer to any and all questions because you're Sherlock'. With Tony I just strut around a little more.""