Robert Downey Jr. upset the world's foremost Sherlock Holmes expert by calling the super sleuth's sidekick by his first name in his new movie.

The actor befriended Malibu, California-based tax lawyer Leslie Klinger while preparing to play Holmes in Guy Ritchie's sequel Game of Shadows but they fell out when the know-all saw the film for the first time.

Downey, Jr says, "I guess I dropped the ball. He was very upset... I said, 'Les, what happened?' He said, 'You called him John!' I was like, 'John Watson, that's his name...'

"He goes, 'He would never call him John. Only call him Watson or Mr. Watson...' It didn't feel right when I said it; I should have trusted my instinct."

And the actor insists he has taken Klinger's comments about his portrayal very seriously: "Thank God there are experts. People take (Arthur Conan) Doyle's interpretations of these characters so seriously. They're fantastic books."