'Harry Potter' Star Spills on Acting Secrets

"If you come visit a set, you might think we're being unprofessional because we're messing around and behaving like little kids," Harry Potter star Jason Isaacs says. "Maybe it happens with European actors more," the British actor adds. Isaacs, known for his role as malicious Lucius Malfoy in the wildly popular Harry Potter series, is referencing his behavior on set of the upcoming drama, Stockholm, Pennsylvania . "Acting is a child-like thing," he argues, and his 20-year-old co-star agrees.

"The thing that I miss most when I finish a film is just the atmosphere that you'll have on set," Saoirse Ronan says. "When I started working, on the very first thing that I ever did, I was only on it for like two weeks or something, but I got so into it, and I loved it so much, I was devastated by the end. It becomes your world."

The two European actors were at Sundance Film Festival promoting their film for the first time, and shared their tips for making a performance, like the ones seen in Stockholm, believable.

"You have to keep your emotions close to the surface. That's why actors, the ones I like, behave like children on set, because when they call 'Action!' they can instantly go into something very quickly," Isaacs says. "And they can let it go just as quickly. You feel the extremity of it. You can let it possess you if you create this open space for it to flow through."

"The relationship that an actor has with the camera, with the lense, is the most important one an actor has on set really," Ronan advises. "I feel like you're telling your story to one little thing."

Issacs echoes his co-stars claims: "The camera loves secrets," he explains. "Nobody wants to watch characters without conflict. One of the great things about storytelling is that we are never alone in our secrets, we aren't the only ones who have them."

In Stockholm, Isaacs plays a man who abducted and raised a small girl (Ronan) before she is rescued and brought back to her biological character. But even in an abductor, there is relatability: "Everyone has something in common with every well drawn and well played character," Isaacs assures. "There's a generation of actors who've been told to do nothing on camera. And that's simply wrong. It's spectacularly boring. Acting is sharing, but calibrating how much you share."

Ronan, known for her roles in films like The Grand Budapest Hotel, The Lovely Bones, Hanna, and 2007's Atonement, is drawn to characters thrust out of their element. "I do like the idea that the character is new and alien to an environment. I've always been interested in what goes on behind the surface — the psychology behind a character. It's always interesting when a character goes through a change or the environment around them is fresh," she says.

While both actors have a multitude of theories and advice about acting (it's rare to meet two people who are so generous with their time), Isaacs says something even non-actor types will relate to. "When you watch things on the big screen you realize why we gather to be told stories," he says. "It's important to continue to gather. Watching other people go through big emotional challenges is somehow uplifting, cathartic. The fact that you're with other people watching them go through it makes you feel more human. It makes you feel less alone. It gives your soul a stir. I see that storytelling has value. That's why it's universal. Acting and story-telling is about connecting."

Images: Getty