He had advertised in a local newspaper for people who lost their jobs to come forth and tell their story for a documentary, he explained.
"We got a heartbreaking response," he said. "Of these, we interviewed 100 people, we put 60 on film and 25 are in the finished movie and are also now members of SAG (Screen Actors Guild)."
Based on the novel by Walter Kirn, the dark comedy about corporate America and male mid-life crisis stars George Clooney as Ryan Bingham who is hired by downsizing firms to give workers pink slips.
It also stars Jason Bateman, Anna Kendrick and Vera Farmiga.
"When I started writing this movie (in 2003), we were in the midst of an economic boom, and I was writing it more as a satire, but in the six years it took me to write it obviously the world changed," Reitman said.
"I realized the satirical scenes about getting fired weren't so funny any more," he said, but added it did not work as a drama.
Reitman himself has never lost a job, however he grudgingly admitted to once firing a seven-year-old girl in a television commercial.
The film "is not about people losing their job in real life," he said.
"It's a movie about a man who is looking for purpose in his life. And the most heartbreaking result of losing your job for people in the middle of their lives seems to be searching for purpose."
He noted that a song playing while the film's end credits roll was given to him on a cassette tape by man in his mid-50s who had just lost his job.
Reitman receives weekly unsolicited music submissions since the release of "Juno" and its acclaimed soundtrack, he said.
"It wasn't the greatest song ever written," he said. But it was "authentic."
The US unemployment rate rose to 9.7 percent in August, according to government data.
--------------------------------------------------------------------------------------------------------------------------------