The 27-year-old actress - who recently split from her boyfriend of eight years Macaulay Culkin - admitted seeing her co-star in the nude during their sex scenes was a highlight of the shoot.
She said: "I enjoyed watching his tushy!"
The brunette beauty also confessed that while she was nervous about baring all in front of the 'Friends With Benefits' cast and crew, she felt relatively at ease stripping off in front of Justin.
Mila added to E! News: "I am incredibly self-conscious in real life about every part of my body. Let alone for the whole crew to see and the whole world to see - the last person I cared about was him!"
However, the pair conceded potentially embarrassing sex scenes were made less awkward because they were told to inject humour into the situation.
Justin - who recently split from long-term girlfriend Jessica Biel - explained: "It was a little less awkward for us because the request was to make the scenes funny as possible. We were more going for laughs, so that helped break the ice."