Revealed: Mischa Barton's secret romance with director of her new film

Mail Online has leared that the British-born Hollywood actress has been very quietly dating Brett Simon, the director of her new movie, "The Assassination of a High School President," which debuted at the Sundance Festival on 21 January.

The film, a spoof of social strata in an uptight catholic high school, co-stars Bruce Willis and his actress daughter with Demi Moore, Rumer.

Scroll down for more...

The project didn't win much praise from the notoriously fickle audiences at the prestigious festival, but according to a source Mischa and her director couldn't have cared less.

"This is a major love affair," the source says, adding that Mischa and Brett's relationship has showed no signs of slowing down since the film wrapped.

Previously, Mischa dated her 'The O.C.' co-star Benjamin MacKenzie, and for several years, partying oil heir Brandon Davis.

But it was director Brett who has ultimately won her heart.

The source says: "They became very close right from the start of shooting last summer, and Mischa was pretty insecure about carrying the movie.

"Brett was the one who helped her get over that by spending hours alone with her helping her develop her character and refine her performance. That's how they got together."

Previously, Brett was best known for a series of short films he directed before his 'Assassination' project caught the eye of Hollywood producers.

He also directed the Killers' video for their monster hit "Somebody Told Me."

The source says that video was enough to convince Mischa to sign onto the film.

Even as Brett worked in Los Angeles on the film's final cut last month, he took time away from the editing bay to help Mischa recover from her embarrassing drunk-driving arrest in West Hollywood.

"Brett was the first person she called," the source says of Mischa's arrest.

The couple have been quietly spending nearly every night together for the last several months, the source adds.

And "they didn't spend a night apart" at the festival, the source confides, even though many involved with the movie remain unaware about the true nature of the director and his star's relationship.

The source says: "This is the most serious Mischa's ever been about a guy and he's the most mature man she's ever dated. They're in love and no matter what happens with the movie, this is going to stay serious."