The film starts about half way through the book, entire sub plots are ignored, those that aren't ignored are usually radically changed, and the main plot is radically changed too (beyond just setting it years earlier and changing the marauding frigate to be French rather than American). The film also has several important bits that just aren't in the book.
It's not really worth detailing the differences, they're too great.
I suppose I should say something about the book-as-a-book. Solid historical action, though I'm beginning to think O'Brian was getting a little too eager to throw unexpected subplots in, that neatly get resolved with no long-term consequences. Some of those sub-plots stretch suspension of disbelief too. Still, it's a good fun read, not intellectually challanging, but very absorbing. O'Brian's skill is in totally immersing you in the world of a early 19th C. frigate, in fascinating but not pedantic detail.
O'Brian freely admits that by this point he was playing fast-and-loose with timelines between books, which does strike me as a failing in serial historical novelists. It feels odd knowing that the characters seem to be perpetually stuck some time around mid 1812 to early 1814 with many long voyages occuring, but no real time passing. Sort of like the Famous Five going on summer holiday after summer holiday but never really getting older. The characters' personalities seem to be stagnating too, since the early books they have developed, but the interplay is staying the same. Between Maturin and Aubrey, depending on the situation, one is predictably the straight-man and the other fall-guy. It becomes quite irritating when the otherwise highly intelligent Dr. Maturin fails to distinguish starboard from larboard, despite having been at sea for many years, and 10 books.
