The handsome actor couldn't stop telling dirty jokes while shooting 'The Ugly Truth' because he wanted to remain in character for another film he was involved in.
He said: "I was doing another movie at the same time where I was very dark and brooding. At night I would go to dinner with everyone and tell dirty jokes. I was vulgar."
Gerard - who has recently been linked to Jennifer Aniston, his co-star in 'The Baster' - also admitted he felt tense when he filmed sex scenes with Katherine Heigl for 'The Ugly Truth'.
He said: "I was so nervous. I kept forgetting my lines. Once, in the middle of a take, I said to Katie, 'Did I touch your breast?' It was an accidental brush but I was pretty nervous about it."
Although Gerard is widely regarded as a heartthrob, the Scottish star recently claimed he thought people were "insane" for calling him "sexy".
He said: "I do tend to get asked to take my clothes off in films quite a lot. Trust me, if I could leave my clothes on in a movie, I'd rather do that any time. But if I'm going to get undressed I'm going to do it as well as I can. Whenever I hear people say that's sexy... for the few people that say that, I think, 'Are you insane?' "
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