Robert Downey Jr. was "pretty terrified" filming scenes with Jared Harris in Sherlock Holmes: A Game of Shadows.

The 46-year-old actor - who reprises his role as the fictional British sleuth in the movie - felt pressure working with the 50-year-old star due to "complicated camera shots," but he believes his co-star's determination to pull off a great portrayal of criminal mastermind Professor James Moriarty is key to the film doing well.

"The stakes were so high, in every scene, and there were complicated camera shots. It's pretty terrifying. What Jared kept pushing toward wasn't personal. It wasn't like, 'I don't want to be embarrassed. I want to do a good job. I want to come off great. I want great dialogue.' It kept going back to this archetype that he was trying to represent. Everything Jared Harris did, in the course of making this movie, was essentially thrown at him with very little time to prepare. It was shock and awe. I think what he brought with him was something that was just so particularly him, while still being this character. It honestly is the main reason that the movie works, but it was also an exercise in trial by fire for him."

Robert has to transform into a woman for part of the movie - which sees Sherlock and Dr. John Watson (Jude Law) join forces to try to outwit Professor Moriarty - but he was less than impressed with how he looked as a female: "I just put on some make-up. I thought I looked like Arnold Schwarzenegger's dwarf brother, or the lead singer of The Cure, Robert Smith."