Oswalt explained that both Theron and Hamm have good looks as well as razor-sharp senses of humour.
"I'm friends with Jon Hamm. Handsome and funny, you don't get to have both," USA Today quotes Oswalt as saying. "[Theron's] the same way. She has the kind of humour that someone who looks like me has."
Oswalt also suggested that Theron put aside her own vanity for Young Adult, particularly in a scene where her character Mavis is recovering from a night of partying.
"She wanted to show what she looked like in the morning, with no make-up, with a bit of a gut, guzzling Diet Coke," laughed Oswalt.
Theron recently revealed that she has very little in common with the former high school 'mean girl' she plays in Young Adult.
Director Jason Reitman's Young Adult opens on wide release in the US on December 16 and on February 10, 2012 in the UK.
> Charlize Theron on career hiatus: "I just couldn't get a job"
Watch the Young Adult trailer below: