“I had one tiny piece of information right at the beginning,” he revealed. “Because I said to her, ‘You have to tell me something, or else I can’t play this.’ It didn’t tell me how to play him, but it told me who he was a little bit.”

So what did she give up? He won’t say. “I swore to her I never would tell until it’s all done and dusted,” he said. Was it the end? “I never knew what the end was.” Was it who Snape really was? “I never knew who he was.” Was it about … Lily? “I’m not telling you anything!” he said — and was that perhaps a sense of glee we noted? Glee because we hit on a right note, or because he was having so much fun keeping the secret?

But he’s got to be looking forward to “Half-Blood Prince,” right? “Well,” he relented with a sneaky smile, “maybe.”

Having read to the end of the finale, Harry Potter and the Deathly Hallows, Rickman says "the only thing I'll say is that for the first time shooting these films, I know what I'm doing and why.

"It's a unique experience," he says of the Potter films.

"Where else is film history going to watch three kids grow up, and a series of films grow up with them?

"I'm very proud to be involved with it."