Robert Downey Jr. upset the Sherlock Holmes expert on his latest film by being too informal.

The 'Sherlock Holmes: A Game of Shadows' star offended the continuity director while filming by constantly referring to his sidekick, played by Jude Law, by his first name - a move which would very be uncharacteristic of the upper class fictional English detective.

Robert told TV host Jimmy Kimmel: "We have a man on the set who makes sure you are being accurate to the book. It turns out the quintessential Sherlock Holmes expert is a tax lawyer who lives in Malibu.

"After Sherlock two I guess I dropped the ball, he was very upset, I said, 'What happened?' and he said, You called him John.'

"So I said, 'Well that's his name, John Watson?' and he goes, 'He would never call him John. Only call him Watson or Mr. Watson.'"

Robert, 46, also did most of his own stunts for the Guy Ritchie-directed movie and didn't get injured, which bodes well for when he plays Sherlock Holmes in a third movie.

He said: "I received no injuries this time . that bodes well for next time. But there were no pranks, there was no time for pranks. We were just beating each other up a lot."