The Juno director revealed that Theron was almost too busy to shoot the Diablo Cody-penned movie, adding that he would not have worked on the film without her.
"Yeah, she was making Mad Max and then she wasn't available," Reitman told The Playlist. "And thank god it worked out, because I wouldn't have made the movie without her.
"I was very conscious of the fact that I was not going to make the film if she was not going to star in it."
Reitman went on to speak about how he met Theron, revealing that the actress approached him at the Oscars.
"We met at the Oscars," he recalled. "She came up to me and told me she wanted to work with me. I couldn't be more flattered, intimidated, and I read the script and almost immediately after I saw her at a restaurant and I went up to her and said, 'I found a movie for us', and she read it and it spooked her a little but in a good way."
Theron recently spoke about her absence from movies over the past few years, joking that she was unable to find a job.
Earlier this month, the actress's Young Adult co-star Patton Oswalt compared her to Mad Men star Jon Hamm, saying she has a razor-sharp sense of humour.
Young Adult opens on wide release in the US on December 16 and on February 10, 2012 in the UK.
Watch the Young Adult trailer below: