hhhhmmm snape in jammies. Sound good. Probably all black and only wearing trousers and no top.

I found this nice report of AR at a charity showing of POA.
YCTV Charity Screening
and Q/A with Alan Rickman
London, 3rd June 2004
" After watching Harry Potter and the Prisoner of Azkaban for the first time on Monday afternoon in the huge Odeon Cinema on Leicester Square, we went to the YCTV (Youth Culture TV) Charity Screening to watch the movie again on Thursday. A special surprise guest was announced to join us after the screening to discuss the movie and answer questions from the audience.
It turned out to be Alan Rickman - Professor Snape - who first apologized to all the kids in the cinema that he hadn't been able to bring Harry, Hermione and Ron along with him. He explained that Daniel Radcliffe couldn't come (which was a big disappointment for my daughter) because he was already shooting the next movie, Goblet of Fire. Emma Watson and Rupert Grint had school exams the very next day (aah! being a teenage film star doesn't mean exception from this torture that every kid has to go through *g*).
So... it was Professor Snape's duty to answer all these questions from a mostly juvenile audience, and Mr. Rickman did his best, sometimes needing help from the YCTV moderator when he didn't get all the Harry Potter details straight.
He was asked how much he was involved in how the Snape scenes were shot in PoA and he explained that indeed he had a good discussion about that with Alfonso Cuaron. In the scene where Snape arrives in the DADA classroom as replacement for Lupin, it was Alan's idea to shut the windows with his wand - which gives the scene an even more impressive effect.
He explained that he had invented Snape in the first movie and was involved from the beginning in discussions about how he would look etc. "He takes over your life a bit." Looking for hints in the books how to play Snape, he liked JK Rowling's description of him barely speaking above a whisper.
He said the scene in the Shrieking Shack took 10 days to film, and after every take they had to rearrange all the dust on the floor to wipe out the footprints and drag marks.
He was asked by a boy about his favourite scene when Snape gets angry with Harry. He said that it was the very first scene in the Philosopher Stone. And he did his best "Mr. Potter" for a fascinated, cheering audience.
Asked if he likes to play bad guys, he told us that he was just looking for good parts in a movie, and if the bad guy had the good part, that it was fine with him.
He then was asked if Snape is in the next HP movie? His answer in best Rickman manner: "I'm afraid so." *lol* And his advise for someone who wanted to know a bit more about the GoF movie: "Go and read the book!"
If you've got the impression, that shooting the Harry Potter movies is only some sort of ordeal for Mr. Rickman, don't be too fast with your judgement ) He told us that it's nice filming the movies because it was like a big meeting of old friends. The adult actors in the movies "had been growing up in theatre together". Alan had been at the Royal Shakespeare Company with Richard Griffith (Uncle Vernon) and David Bradley (Filch), and he has done some movies with Emma Thompson (Professor Trelawney).
I think, taking part in a Potter movie, is some sort of „royal duty" for Britains finest actors. And we are more than grateful to them. J
After the Q/A session there was a dinner buffet arranged for the charity supporters and Alan Rickman stayed for another half an hour, patiently signing autographs for everybody, young and old."