He was in the 'John Malkovich school' of actors. Someone whose face many might recognise, but whose name they might not know.
I'm a bit of a fan of his - he won a Best Actor Oscar a few years ago, for his portrayal of Truman Capote in the film 'Capote', and he was excellent in Happiness.
Just heard on Radio 4 that Seymour was found with a hypodermic needle in his arm and that packets of heroin were lying near-by. Too early to be exact I suppose but it appears to be the usual reason...drug addiction.