Charlize Theron's co-star was "astonished" by the actress' performance in their newest film.

The 36-year-old actress stars in the upcoming comedy-drama Young Adult. Charlize plays divorced woman Mavis Gary, who returns to her small hometown to win back her happily married high school sweetheart.

Patton Oswalt, who portrays Charlize's character Mavis' former classmate Matt Freehauf in the feature, was deeply impressed by her technique.

"What Charlize does in this movie you never see this in other movies," Patton told ET Online.

Charlize won an Academy Award for Best Actress for her performance in gritty drama Monster, which was released in 2003. The film followed the grim life of American serial killer Aileen Wuornos.

Patton believes Charlize has reached new heights in her talents in Young Adult.

"I think emotionally, this is way rawer than the stuff she did in Monster which was also brilliant, but in Monster she doesn't look like herself so now there is no artifice," Patton explained.

"She is channeling this other thing through her without judgment and it's kind of astonishing to watch it."

Young Adult will be released in US theatres on December 16.