[Aiwendil]

Peter Jackson seems to interpret the Gollum-Smeagol dichotomy as a true example of 'dissociative identity disorder' (formerly 'multiple personality disorder'). The sense I got from the films was that Gollum and Smeagol were to be thought of as two distinct minds, able to converse with one another but each ultimately self-contained, with its own thoughts and will.

I don't think that the book necessarily excludes that situation as an interpretation, but it also does not necessitate that interpretation, nor even particularly suggest it. My understanding prior to seeing the movies was always that Gollum was merely ambivalent and that the two 'personalities' were not much more distinct than the competing impulses in the mind of anyone who is ambivalent about something.