I saw this because Johnny Depp was in it. Sometimes it's fun to see films and try to figure out why they attracted him.
It's easy to see in this case, since it's a wonderful part in a very interesting film. He plays Dean Corso, a unscrupulous book dealer (a "book detective," as someone calls him). In the first scene he flimflams some unsuspecting civilians, and then he puts the screws to the friend who will help him unload the rare volumes he just acquired by chicanery, refusing to share more than 10% of his windfall.
Then he is hired by a rich man named Boris Balkan (Frank Langella, not as beautiful as he was when young but an even better actor now) who is obsessed with the devil. He has a copy of a book supposedly co-written by Lucifer several centuries ago, and he wants Corso to take it and somehow compare it to the other two surviving copies, because he's convinced only one of the three copies is genuine. The true copy, correctly employed, can supposedly be used to conjure the devil. Balkan's copy is apparently not functioning as advertised.
In the course of Corso's travels, he deals with the widow of the man who sold the book to Balkan (immediately before hanging himself), and the owners of the other two copies.
He also acquires a helper, a young and attractive woman (the credits just call her "The Girl"), played by Emmanuelle Seigner. She's apparently a student, and she trails after Corso and protects him. She has some nifty martial arts moves.
She can also fly, which we see but Corso does not.
The important fact, I believe, is that this is a movie about god and the devil made by someone who doesn't believe in either (Polanski makes this clear in the commentary). So, this is an intellectual exercise in answering the question of how the struggle between good and evil would play itself out.
The key to the movie is that The Girl is the devil, or at least a manifestation. She is looking to collect Corso's soul. She never appears to the Satanists who try to summon her because they are already hers, already on her side of the ledger. Why should she waste time with them? The ones who interest her are the ones on the fence, the undecided votes, as it were.
Corso is not evil, but he is unscrupulous, eager to gain any advantage in his business. And he's definitely willing to have sex with a beautiful widow even though he knows she's just looking for the book (and that he's stashed it where she can't get at it).
The Girl wants to keep him safe while she seduces him more and more into evil. And she appears in a form that he would find really attractive, not a bombshell like the widow (Lena Olin) but a student in jeans and sneakers and mismatched socks, pretty much a female version of Corso himself.
Evidence? One of the first times Corso talks to her, trying to find out why she keeps showing up wherever he goes, what is she reading? How to Win Friends and Influence People. Which is what she's doing, of course.
Nothing in this movie is casual; everything is there for a reason. When Corso sees that a friend of his is dead, in a very striking pose, he says, "Oh, Jesus." At that point, he has not yet crossed over. However, when he first sees the clues that link the three copies of the book, he says, "I'll be damned." Which is literally true. But then, when he sees another corpse, he says, "God almighty." Still on the fence.
When The Girl has got a bloody nose defending Corso, she takes her blood and marks him with it. He does not immediately wipe it off.
Later, Corso and The Girl are being held by the henchman of one of the villains. She holds back (from what we've seen, she could have dispatched him with little effort) and lets Corso get the better of him. He then beats the man again and again, with a fury we haven't seen from him before. The Girl watches eagerly, then she says, "I didn't know you had it in you."
I could cite a lot more examples, but you get the picture.
(I will mention one, which just occurred to me, and which could be a coincidence, but I don't think it is. The first time Corso confronts The Girl, demanding to know what she's up to. He asks her name, and she says, "Guess." Which reminds me of this:
Pleased to meet you
Hope you guess my name.
But what's puzzling you
Is the nature of my game.
Which is, of course, from "Sympathy for the Devil.")
The great thing is that none of this is telegraphed, there are no big signs saying how significant these things are, they are just part of the story. But, like the patterns in the three books, they all fit together when you know what to look for.
People complained that the ending was anticlimactic. But it is both final and inevitable, and (unlike many movies) not marred by a lot of unnecessary special effects and hoopla. Just a man making a choice.
As Johnny Depp says, speaking of Corso: when you meet him, you don't like him. He's bad. But later, when you do get to like him, he's worse.
The other thing which is particularly enjoyable about the film (and which was true of The Ghost Writer as well) is that Polanski uses a lot of CGI, and his use of it is both subtle and witty (two words not usually used in connection with CGI).
Most of the effects are designed to be invisible, many of them to enable him to work in the studio rather than going on location, but one particularly enjoyable bit is when Corso goes to see two elderly book binders (the ones who sold the copy of the book which ended up with Balkan).
They are twins, and played by the same actor, occupying the same set via motion control, and their interaction is a lot of fun to watch (even more when you learn that José López Rodero, who played the parts, is not an actor at all; he's a second unit director and production manager). Then, when Corso goes back to their store at the end, the brothers are gone and two workmen are taking the place apart. The two workmen also played by Rodero. Hitchcock would have enjoyed the whole thing.