Robert Downey Jr. once wore a monkey mask to stop people knowing he was "blitzed".

The 46-year-old actor - who was arrested a number of times on drug-related charges from 1996 to 2001 - admits he was once so high he put on a 'Planet Of The Apes' disguise so no one would know who he was.

He said: "About 20 years ago I went on a tear, and I put on a 'Planet of the Apes' mask that would keep people from knowing how blitzed I was. It worked for about the first 15 seconds, 'It's me, Cornelius!' "

Now on the straight and narrow, Robert is starring in 'Sherlock Holmes: A Game of Shadows', the sequel to the 2009 movie 'Sherlock Holmes', and he admits he had a great time on set with his co-star Jude Law, who plays his sidekick Dr. Watson.

He said: "People who talk about chemistry and what does that really mean. We work really hard, we have an immense respect for each other and we've also seen - and some of us have been in - sequels that sucked, and we wanted to kind of avoid those pitfalls."

Jude agrees, and he particularly relished the chance to go back because everyone knew the on-set workings.

He told BANG Showbiz: "I think also, no matter how happy, harmonious and creative the first film was as a group, it's always true to say that 20 or 30 per cent of a film is taking up getting to know each other, and you end on a high.

"You end knowing how each other works. It never felt like we dropped the ball from the first to the second, we never assumed we were going to make another and there was certainly a lot of enthusiasm for relationships that worked and we wanted to flesh out a little more. So I was excited to investigate and mine more of the same."