Directors waited until two days after the actor had turned 18 to film his saucy scenes with co-star April Pearson - to avoid legal issues.
Hoult says, "They were all laughing and saying that was my birthday present."
But the actor, who turned 20 on Monday (07Dec09), admits it was the greatest birthday gift: "It was odd. April was a very nice girl but the 'cut' thing was awkward - you're doing a passionate sex scene with someone and then I'm lying on top of her and they say 'cut' and you're in a very compromising position with someone you don't know that well.
"Needless to say my parents were not on the set that day. I wouldn't want them to see their son having sex on camera!"