(Cover) - EN Showbiz - Justin Timberlake was lacking in confidence during the Friends with Benefits "ass day".

The 30-year-old singer-and-actor stars alongside Mila Kunis in the comedy about two pals who decide to introduce a sexual element to their relationship.

The movie involves some rather steamy sex sequences, and Justin admits stripping off on camera made him particularly nervous.

"I had an ass' day where I had to run back and forth to the bedroom and show my ass on camera. I felt very insecure that day," he laughed in an interview with Total Film.

The heartthrob also reflected on the notion in the picture that men are more motivated by sexual endeavours than women. Justin argues that males cannot control their passionate urges, although is quick to point out that generalisations shouldn't be made.

"We're chemically built differently. I think men compartmentalise life and women connect life. Not every guy is like that but it's more true than not!" he smiled.

Justin was joined in the interview by his co-star Mila, with reports that the pair are romantically linked recently surfacing. The couple insist they are only pals, and Mila is eager to point out that she and Justin remained professional during their sex sequences.

"You really can," she replied when asked if she and Justin managed to maintain eye contact during steamy scenes.

"I mean, you'd have [someone] running in and fixing blankets while we'd be holding positions. It all became very surreal." (C) Cover Media