This is funny. I read the book before I watched the film and didn't know that the movie was released before the book was published. I was amazed of how well the movie put images to what I had read in the book, and having read it, I didn't find it difficult to understand the movie, I just thought it was one of the best adaptations of a book I'd ever seen!
Yes, having read the book makes a huge difference. I tried watching the film first, but I failed and stopped watching a short way into it. Then at some point I read the book, which I really enjoyed. I thought I'd give the film a second chance and I'm glad I did because it was brilliant, having read the book.
Strongly recommend treating them as two integral parts of one work of art -- consuming either one and not the other will leave you much less satisfied.
I think that may be true for several other longer adaptations including STALKER (from A Roadside Picnic) and Apocalypse Now (from A Heart of Darkness).
Stalker is interesting in that Tarkovsky originally used the Strugatskys' screenplay, but went back and changed it.
Tarkovsky had become increasingly disillusioned about the whole project as he entered into production — he had fights with his cinematographer and had doubts about the story. At one point, they had filmed 6 months of material when they discovered the film negatives were unusable, for reasons nobody has been able to figure out (they used Kodak stock imported from the US, which Soviet labs were unfamiliar with, and it may have been out of date; Tarkovsky himself suspected it was sabotage). As a result, the project went into hiatus while Tarkovsky considered his next steps and sought new funding from the Soviet film board to reshoot. It was actually Arkady Strugatsky, after many discussions and drafts, who suggested a compromise, where most of the sci-fi elements would be stripped from the film, and he encouraged Tarkovsky to build up the screenplay again based on his own ideas.
It's fascinating to think what Stalker might have been, if Tarkovsky hadn't deviated from the original script. Roadside Picnic is certainly a novel that could work as a film or miniseries. (WGN considered producing a TV show based on Roadside Picnic last year, with Matthew Goode as the lead, but the pilot was rejected.)
Heart of Darkness/Apocalypse Now also has a pretty remarkable adaptation in video game format called Spec Ops: The Line. It's the same story premise cast as a modern military shooter. The gameplay is a fairly generic (some say this is an intentional statement on the drudgery of video game violence but I'm not so sure) but the story is excellent and there are lots of audible and visual details that show the gradual descent into madness of the main character and his teammates -- your voice lines go from professional callouts to angry emotional outbursts, "Hunt him down!", "Got the fucker!" etc.
On the other hand, I read the book before seeing the film and wish I had done it the other way around. Same case with The Martian too - I found myself distractedly comparing notes (in my head) too often while watching.
Perhaps the one book-then-movie reading/viewing sequence I'm happy with was Casino Royale. It was a good faithfully-gritty adaptation.
