Movie star Robert Downey Jr's latest movie kissing scene was one of the most difficult moments of his career - because he had to lock lips with a smelly Val Kilmer.

The pair play an unlikely duo in new crime comedy Kiss Kiss, BANG BANG, but Downey Jr admits he can't watch the movie because there's one scene that turns his stomach.

He explains, "Val had just had a hoagie. I smelled German deli mustard. There was nothing that wasn't coming out of his gullet when he kissed me.

"Ironically, all day he was saying that my breath stank and he was feeding me Altoids (mints). Eventually, I said, `Don't come at me like some feasting pirate. It was horrible.'"