The Water For Elephants actor revealed that he felt uncomfortable disrobing on camera for his character Edward Cullen's intimate Honeymoon scene with Bella Swan (Kristen Stewart).
In an interview with Mexican magazine 15a20 - via fan site Twilight Poison - Pattinson explained: "You also have to worry about how you're going to look. I had never been so worried about that before, but after seeing how much Taylor [Lautner] works out for the movies, I thought, 'Well, everybody is really going to judge me now.'
"I always tried to think about positions where I could tense the muscles, and suck in my belly. I think I just gave the most vain answer I could come up with."
Pattinson added that he followed a strict diet and exercise regime for nearly six months in order to prepare for the part, saying: "In this movie, I thought, 'Okay, it's the last one, I'm getting a six pack,' and I tried, but it's very hard.
"I worked out so hard for six months, so after filming the sex scene, the first thing I thought was, 'Time to eat!'"
Stewart also recently admitted that she felt "uncomfortable" shooting the scene, as she felt there were high expectations for the characters' intimate encounter.
The Twilight Saga: Breaking Dawn - Part 1 will open in cinemas on November 18, followed by Breaking Dawn - Part 2 on November 16, 2012.
Watch a trailer for The Twilight Saga: Breaking Dawn - Part 1 below: