Let's face it - the man IS Snape when he plays that character - brilliant! So, I'm emailing Ninq about him and I think to myself: How are they keeping this guy as Snape? He must be offered so many roles, and this has to get in the way.
So I went to imdb.com, and found some interesting facts: (LJ cut for length)
He was born to Irish and Welsh parents in 1946. He attended the Royal College of Art; he wanted to be a graphic artist. At age 26 he won a scholarship to the Royal Academy of Dramatic Art where he spent 3 years. Started out in the theatrical version of "Dangerous Liaisons" His first major film was Die Hard (1988) where he was cast as Hans Gruber.
Has been in a relationship with Rima Horton since the 1960s but the two don't live together, as she has to stay in the district she represents in order to retain her Council seat. Alan live in another part of the city.
Although a highly successful film actor, he has frequently passed up film offers to return to the theater. (A place and activity which he calls "magical" and his "first love".)
When he was a child, he had a speech disability: his lower jaw was very tight, causing his words to be indistinctive and muffled.
For the shot where Hans Gruber falls from the top of the building in Die Hard (1988), was actually dropped by a stuntman from a 20-foot high model onto an air bag. To get a genuine surprised look, the stunt man dropped him on the count of two instead of three.
In Die Hard (1988), he is seen firing a gun only once (in the atrium when Gruber realizes Holly is John's wife), due to his uncontrollable habit of flinching when the gun recoiled. In that scene he blinks just before the camera cuts.
Chosen by Empire magazine as one of the 100 Sexiest Stars in film history (#34). 
Says Canada is like a second home.
Failed his California driving test at one point for "driving too cautiously through a green light" [*ggl*}
But I never did answer my original question. He's been doing only about 1 movie a year, in addition to HP... but he's may also have been doing plays, so...!