Blake Lively's assignment the first day she had to shoot the crime-drama The Town: A sex scene with Ben Affleck!

In the September issue of Interview, Affleck, 38, tells Lively, 22, he was impressed she was "very poised and comfortable about it - more mature than some of the crew members."

Lively's secret?

"Well, I have a big family and no one ever treated me like a child," she explains. "They always treated me like an adult, like an equal. But with acting, I think half of it is just acting confident. We stand on these red carpets and pose in these dresses, but we're all only so confident. It doesn't mean we think we're great looking or anything. It's all a facade. But the sex scene was really awkward. It was my first day, so thank you for that."

The scene was beneficial in one way.

Says Lively: "I guess it was good to break the ice."

The Town hits theaters Sept. 18.