On being told about Snape by JKR prior to filming the first Harry Potter film:
Alan: "It was a small piece of information that she let me know that there was more to him that met the eye." Patrick: "Can you share with us what that was?" Alan: "No, I told her I never would. And I never have."
That's just so right.
He also talks about his involvement in developing Snape's costume: He insisted on buttons, buttons, buttons. Clearly, he understood the character.
Lots of other interesting stuff - very funny & articulate. Squeeeee!
ETA: Belated Happy Birthday, Severus Snape (January 9)!