Ben Stiller has praised his Tropic Thunder co-star Robert Downey Jr, calling him a 'genius'.

Speaking to Access Hollywood, Stiller said of the Iron Man star: "He's brilliant. I think he's probably one of the best actors I've ever worked with, if not the best".

Stiller and Downey Jr are joined by a number of high-profile comedians in the Vietnam war movie spoof, including Jack Black and Steve Coogan.

But it's Downey Jr's portrayal of Kirk Lazarus, an actor who goes to extreme lengths to acquire a part in the film, including pigmenting his skin so he can play an African American, which draws the greatest praise from Stiller.

"I'm having this experience of watching him and improvise and take this character which isn't an easy proposition to make this guy relatable, and he's just amazing," he said.

"I just think it's amazing seeing him have the respect and success at this point in his life after everything he's been through. He deserves it. So I was really excited to work with him."

Stiller, who co-wrote, directed and stars in the film admits the character could potentially be seen as being controversial.

"I felt we had to be really clear about what our point of view was where the humour was coming from with this movie," he added.

"We were satirising actors and the lengths actors go to for recognition and attention."

Tropic Thunder is released in the UK on September 19th.

09/08/2008 10:51:11