I've just been re-reading the book, and perhaps one of the more striking things about the film is what they left out. In the book, Morvern's foster father plays a very large role, and much of the middle section revolves around her somewhat uneasy section between his new partner and Morvern. I'm not sure whether it would made Morvern more sympathetic if this was a focus in the film, perhaps, in an odd way, it would've made her more unknowable- which is to say, how can she react so callously to the suicide of her boyfriend when she's clearly capable of warmth and intimacy with her foster dad? Also, the suicide note in the book is much longer, and essentially puts all the blame on himself and says she's a good person, whereas the note in the film is fundamentally patronising ("you wouldn't understand"). Again, it's hard to know whether this changes our relationship to Morvern or not, perhaps it would've made her even harder to relate to.
Also, you find out a lot about their backstory in the book- it turns out he's eight years older than her, and they got together when she was 16, after working at the supermarket after dropping out of school. Morvern, at the start of the book, is 22. Apparently, he largely had a good relationship with the foster dad, too. In the film you've got no idea how long they'd been together, and what the situation was that brought them there. Which again, has the effect of making the film more of a blank slate to draw on.
Finally, perhaps the most surprising divergence is in the respective conclusions of the novel and the film. In the very last line of the novel we discover that Morvern became pregnant after sleeping with someone after a rave, in the film, it's left out in the open as to what is going to happen next to her. Why did Ramsay decide to do away with this revelation altogether? Is it because this would seem like too big a deal after what has largely been a very stripped-back, minimalist narrative?
I think what the film captures of the book very well is its essentially unknowable quality, its "glitchy" prose and associative narrative, as I put it. What it does is complicate this even further by parsing back the narrative to its absolute bare bones, regarding even Morvern's backstory as an unnecessary complication. Above all, it captures the novel's sense of both drawing the reader right into the story (its prose is sometimes wincingly intimate) and pushing them far away (we can never quite figure out where Morvern stands, or what she's thinking). It's hell of an achievement.
Ironically, the area in which the film is most faithful is in its music selection-many of the songs played in the film were already in the book- Morvern's constantly making mixtapes in the novel, and so there are quite a few different tracklists throughout. The book, incidentally, is dedicated to Holger Czukay of Can.
I realise I'm concentrating on the differences between the source material and the film here, but I feel they're interesting, because it shows how Ramsay went even further than the novel in ommitting things. I liked snarfy's comparison to Antonioni's the Passenger- the film did remind me of Antonioni's own approach to filmmaking at times, not least the long, aloof takes which draw you into a particular moment and pull you away once you realise the director isn't going to give you any visual clues as to how to feel towards the scene, rather you're meant to just absorb the visuals. There's the same element in the performances, too, think of the closed-off nature of Morvern and her roleplaying throughout the film. (Similar to how Nicholson's character assumes the identity of a supposedly dead man in the Passenger)
It's before my time but I've been told, he never came back from Karangahape Road.