The 46-year-old revealed that he was happy to reunite with co-star Jude Law for the follow-up to 2009's Sherlock Holmes, Sherlock Holmes: A Game of Shadows.
"Well, ever since Mr Law and I met, the chuckles, and the fun, and the happiness come," Downey Jr told Collider. "But we are really quite about the business very much, I believe, as Holmes and Watson are."
He continued: "So... it's 99% perspiration. Oh, you are going to love this - we like getting sweaty together. I mean, we like working our asses off. We like to work."
Sherlock Holmes: A Game of Shadows will see Downey Jr reprise his role as the titular detective, with Law playing his trusty sidekick Dr Watson.
It was recently reported that a third Sherlock Holmes movie is in the works, with Warner Bros hiring writer Drew Pearce to pen a second sequel.
In October, Downey Jr said that his role in Iron Man changed his life, revealing that the first instalment in the comic book franchise helped him to get his career back on track.
Sherlock Holmes: A Game of Shadows opens in US and UK cinemas on December 16.
Watch the special content trailer for Sherlock Holmes: A Game of Shadows below: