Originally Posted by kane

Actually a lot of the time with the middle name thing it is about the SAG union. They can only have one person with a specific name in the union. If there was already a Phillip Hoffman they may have made him use his middle name. Then, legally, any film has to bill him as Phillip Seymour Hoffman.

I don't know if that is the case with him, but I know with some actors it is. It is why Michael J. Fox uses the J.