For Jeremy Renner's 40th birthday party last month, the Oscar nominee ('The Town' and 'The Hurt Locker') mixed family and celebrity for the first time. Famous attendees included Charlize Theron and ScarJo.

Renner explained on 'Jimmy Kimmel Live' (weeknights, 12AM ET on ABC) that he was fearful a relative, unaccustomed to schmoozing with some of Hollywood's glitterati, "might drink too much and do something very terrible." The host elaborated helpfully: "Uncle Deon has a few too many, starts smacking Charlize Theron on the ass."

But everyone was composed -- though rumor has it Christina Aguilera spent the night in Renner's bedroom. True? "No, she was... hanging out. We were eating cookies and milk." Uh-huh.

Watch Jeremy Renner's full interview here: