They're actors. They have to practice their lines, including ridiculously long words such as raxacoricofallapatorius (I totally had to add this to my spell check dictionary a while ago, because I can't spell real words properly, let alone this!)
I love hearing him say those words like it's nothing. I mean, he is a Timelord, and apparently he possesses nearly infinite knowledge of the universe about him, so it would make sense that he can say these things and know what they are. Still though, it's just another thing about the Doctor that's ******** hilarious.