From IRK Magazine:

Chris Knight: Robert Pattinson once said the best piece of acting advice he ever received was from you, about how you always keep your ‘birthday in your back pocket’ when you’re in a scene. Can you talk a little bit about where that idea comes from and some of the most interested ways it’s manifested itself in a character for you?

Peter: That was actually an inside joke that I think people misinterpreted. When you’re an actor, teachers suggest you hold a secret in a scene to give you something interesting to play. We were talking about this one night, and I jokingly said to Rob that my secret is that it’s always my birthday. And that it works for any scene. If you’re sad in the scene, you can play “no one remembered my birthday,” If your excited, “hey, it’s my birthday,” If your jealous, “my birthday party is on the the same day as my best friends,” If you’re angry, “you hated your birthday presents,” I joked that it works for any emotion and it’s a secret no one else would know– and you could keep it to yourself. He laughed. Then said, “Actually… that’s pretty genius.” Haha. But it was kind of an inside (actor’s) joke, so when he explained it in an interview I think people took it literally.

Thanks @sallyvg for heads up.