London born actress Mischa Barton has been secretly dating the director of her latest project, Brett Simon. The movie, The Assassination of a High School President, recently debuted at the Sundance Film Festival and also stars Bruce Willis and his daughter Rumer.

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.”

Barton has been previously linked to the notorious oily heir Brandon Davis and also Cisco Adler.

photo source: WireImages

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