It (I will not call it the Clown for these purposes) in the film is a much more limited Bogie than in the literature. This is because all the cosmic and spiritual aspects of the book were cut out. For all intents and purposes the monster in the film is merely that a physical creature hanging out in the sewers. It is, for all intents and purposes, that weird spider/humanoid thing. The filmmakers felt either they couldn't manage that aspect of the story, or didn't feel the average person buying a ticket would get it anyway. There are a host of other differences too, based largely in the way the story is rewritten and encounters changed, but those are more ephemeral. They go to the behavior and motivation of the monster, so it wouldn't be possible to cover them in a meaningful way in a reasonable amount of time.