Though Charlize Theron and Stuart Townsend co-star in Head in the Clouds, out Sept. 17, the couple’s real-life love is seriously grounded. The two showed up, separately, at the posh Regent Beverly Wilshire hotel to promote their new film and took turns spying on one another while each talked about their on- and off-screen relationship. When Theron found out that Townsend, who just arrived in L.A., was listening in on her, she hugged her boyfriend hard, asking, "When did you get in? Why didn’t you call me?" before kissing him deeply and complimenting him on his new haircut.

That’s epic romance.

Head in the Clouds is similarly epic. A romantic drama set in 1930s Europe, the film explores the life of Gilda Bessé (Theron), a woman who strives to continue her hedonistic lifestyle in the face of opposition from her sometimes lover — the shy, yet socially conscious, Guy (Townsend).

"It’s tricky to explain," Theron says of working with her boyfriend. "The reason I thought that Stuart and I could actually bring something from our relationship to this material is because it’s just not a relationship about love. It’s really about the opposite — the destruction of love.

"My character’s the manipulator in many ways; she’s the one that breaks the relationship down every single time. And I think the advantage you have when working with someone you know is that you know how to push their buttons." Theron adds that acting is very personal, and having someone you know really well, acting with you, makes "hurting them a little easier."

What?!

"It’s a terrible thing to say, but it’s true," she confesses. "I found myself having to get him to a place where he was truly affected, his heart being completely ripped out of his chest. Being his girlfriend, [laughs] I had that power. And I had to kind of abuse it a little bit."

Townsend, in his soft Irish accent, laughs when saying that he appreciated the abuse. "She’s a master manipulator — it was nice," he says. "It was nice to be able to have your heart broken by your girlfriend and go home at night and hang out, order room service."

For Theron, there’s a comfort level with Townsend that she was afraid movie audience might not be able to get around. "There’s a fear of it becoming the people that you know as celebrities on screen," she says.

Townsend acknowledges the fear, but claims that the intimacy also makes the love scenes easier. "No matter who I was in the film with, I’d probably imagine Charlize super-imposed over the other actor’s face so I could picture [that’s] it’s Charlize I’m loving," he says.

"But I’m aware and worried that [the] chemistry may not come across on screen. Sure we know each other and love each other, but that doesn’t always translate onto the screen. "But," Townsend adds with a laugh, "most people who see this film won’t know we’re together. We’re not a Ben-J.Lo juggernaut."

Working with your significant other does have some drawbacks, though, according to Townsend. "I never thought it would happen that I’d get tired of Charlize, but film is so intense. During this movie, we went to London for 10 days; I had to stay in Montreal, and we were just like, ‘See you later!’ We spent so much time together, day in and day out. It was nice to have a little break. Every relationship needs breaks."

Theron agrees. "I didn’t come to the set when it was just Stuart," the Oscar winner laughs. "I took my off days. I went shopping!"

Though they appreciate their relationship hiatuses, the couple doesn’t discount the possibility of starring together in another film down the road.

"We’ll work together if we feel [that] us being in a relationship adds something to the material instead of just, ‘Come and watch Charlize Theron and Stuart Townsend onscreen!’ I’d be totally up for it," Theron says. "I love working with him. Putting aside that I’m biased, and he’s my boyfriend, I think he’s a terrific actor."

Head in the Clouds releases in theaters Sept. 17.