LOS ANGELES—You won’t see many Oscar winners born in the ’50s who inject as much fun into a press conference as Emma Thompson. The actor, who took home the Best Actress award from the Academy of Motion Picture Arts and Sciences for Howards End (1992), begins the day at a Beverly Hills hotel by running past her costars out of order and scaling the four-foot barrier to get to her seat in the middle of the podium in front of the press. For comparison’s sake, her Saving Mr. Banks costar Tom Hanks (also an Oscar winner born in the ’50s) respectfully elects to take the stairs.

She spends the rest of the afternoon gleefully doing off-the-cuff impersonations of her character in the film, real-life Mary Poppins author P.L. Travers, and answering questions by going off on a tangent, realizing it, and half apologizing: “I don’t think I’ve answered your question at all, so do forgive me. But it was the [most] interesting thing I thought I could tell you.”

Thompson is generating serious buzz for her performance in Banks, which opens Friday (December 20). She steals every scene she’s in as Travers, the cantankerous author who refuses to give Walt Disney (Hanks) the film rights to her novels. Recognizing that Travers would be tricky to depict, Thompson undertook a lengthy search to understand her. “She was like going into a maze,” says Thompson. “Around some corners you’d find this terrible monster and around another corner you’d find a sort of beaten child. So she was the most extraordinary combination of things and I suppose that was the scariest thing because in films we often get to play people who are emotionally or at least morally consistent in some way. And she wasn’t consistent, in any way. You would not know what you would get from one moment to the next. You could have a very close moment one day—and I got this from her friends—and the next day they would have gone and seen her and she would have treated them with… well, it’s like that moment with Paul Giamatti’s character [Disney’s driver, Ralph] where she says, ‘You’re the only American I’ve ever liked.’ And he says, ‘Oh, why? Can you tell me why?’ And she says, ‘No, I don’t want to tell you any more about that, now you’re just asking too much. Go away.’”

At the same press conference, Banks director John Lee Hancock (The Blind Side) cites working on one of the film’s last scenes, in which Travers silently goes through a number of different emotions, as an example of how hard the actress worked on the part. “You [Thompson] told me ‘I’m not sure where the bridge will be built, but once I know, I can cross it again and again,’” Hancock recalls. “And I thought that was just fascinating, because I’m not an actor but to witness that in terms of ‘I’m not sure how that’s going to happen but once I know how the bricks were laid, how to cross the river, I can go there again and again,’ which she did, which was amazing.”

And while Thompson put a copious amount of work into the character of Travers, it’s also clear that she wouldn’t be joking around and climbing the walls if she had had a terrible experience on the film. On the contrary, one gets every indication that the actor took genuine joy in lending herself to the ugly sides of Travers. “Isn’t it rather nice for all of us, who’ve been so well brought up and are so bloody polite all the time, Americans particularly, to see someone being rude? It’s bliss, isn’t it? I think we act quite a bit of time in conflict with how we really feel.”

Her use of “we” is a bit peculiar, because no one could ever accuse Emma Thompson of such a thing.