Well, I’ve read the novel and I wasn’t entirely sold by the movie’s plot changes, either (although the Texas Hold 'Em was fine by me!)
In the movie, Le Chiffre is a high-level money launderer to international terrorist groups who more or less reports to “Mr. White,” a financier and/or liason with such groups. In the novel, Le Chiffre is described a long-term Soviet operative, an “undercover Paymaster of the ‘Syndicat des Ouvriers d’Alsace,’ the communist-controlled trade union”. So far, so good: the shifting focus from the USSR to terrorist groups and the financiers who abet them is a timely update; especially given the collapse of the USSR and the steady declines of heavy manufacturing and trade-unionism in the west, and 9-11, which is referred to in the film. Casino Royale may be an attempt to re-start the Bond franchise, but it’s still set in the 21rst-century present day.
In the novel, Vesper Lynd has long been compromised by Moscow, which has incarcerated her Czech [IIRC] boyfriend – the deal being, he gets to live (and send her occasional letters as proof of life) as long as she works as a double agent, compromising the identities of western intelligence assets. Note that in this arrangement, the KGB (or SMERSH; I don’t remember how Fleming cast it) has its hooks into both Le Chiffre and Vesper from the start, and has benefitted from these arrangements for a number of years already. This paves the way for a natural linkage between Le Chiffre’s operation and Vesper’s role in undermining any British effort to destroy him.
In the movie, Vesper Lynd is, if I understood correctly, extorted by Mr. White (who apparently has kidnapped or otherwise threatened Lynd’s Algerian boyfriend) to compromise Bond’s attack on Le Chiffre. Dench’s “M” explains at the end in a voiceover coda that Bond’s life was spared in the killing of Le Chiffre by a deal made by Vesper to save his life, at the expense of her own. Here’s where the movie gets into all sorts of plotting problems, IMHO. It wasn’t clear to me whether Vesper had just fallen under Mr. White’s influence or not – but since Le Chiffre’s money troubles were of a precipitous nature, and the British operation to beat him at cards was similarly hasty in its development, how could Mr. White have worked up a means to control this British government official except on the spur of the moment? But this doesn’t seem very possible. But then why would he have bothered to do so from way back when, when he couldn’t have had any way to anticipate how helpful Vesper could eventually prove to him? (Why any official from the Treasury, when there’s MI6 to infiltrate and compromise?) What’s more, Mr. White is a poor substitute for the USSR in a practical way. It’s an all-too-real slice of life to suggest that a government agency as well-funded and staffed as the KGB could cultivate agents and double agents in the west, but it’s a real stretch to imagine that a solitary underworld money man (even one as smooth and well-off as Mr. White) could execute similar coups.
Then there’s the Mathis angle. In the novel, a hapless pair of Bulgarian agents accidentally blow themselves up with a bomb intended for Bond; Bond narrowly escapes death, with Mathis quickly coming to his aid, deflecting the press coverage away from Bond and leading his own investigation of the bombing. Mathis is a most skilled and reliable ally. Unfortunately, the movie minimizes his role (not only does he fail to protect Bond from getting shanghai-ed by a tainted drink in the casino, but all he gets to do is frame the local police chief and plant the bodies of the terrorists who were killed by Bond on an ally of Le Chiffre) and then casts his loyalty in doubt. “Keep sweating him,” Bond advises in the end, but to what end? Surely a French spymaster like Mathis would know all about harsh interrogation methods and how to resist them. If Bond believes Le Chiffre’s boast about Mathis, then he should have advised a covert investigation of Mathis, to unveil his true loyalties and contacts.
Even the torture and recouperation scenes were mishandled. In the novel, Le Chiffre tortures Bond in order to learn steal his [baccarat] winnings, which Bond had carried from the casino in the form of a bank check (which he’d stuck behind the number on the outside of his hotel room door!). Bond is tortured for over ten minutes (of solid beating to his genitals, with a carpet beater) before fainting. He’s revived with coffee, just in time to witness Le Chiffre’s reaction to the arrival of the SMERSH assassin, who disposes of Le Chiffre for his bungling and spares Bond’s life because he doesn’t have orders to kill him as well, but who also brands Bond with an “inverted M” symbol on the back of his right hand. (Since the movie series has ignored this detail, it makes sense to omit it in this film as well.) Bond then slowly recouperates and tentatively rediscovers the joy of sex.
In the film, Le Chiffre is still motivated by the money (this time, to get the password; he expects his thugs will get the account # from Vesper readily enough). He admittedly hits Bond very hard a few times, but only a few. The assassin comes along too soon and kills Le Chiffre for his incompetence, as in the novel, but the assassin here was Mr. White himself, wasn’t it? How would he know where Le Chiffre would take Bond? Was his presence at the casino established earlier in the film? This twist seems contrived to me. This transition if problematic for another reason: because the [duration of the] torture is soft-pedalled (albeit for solid cinematic reasons), Bond’s lengthy recouperation seems a tad too cushy. His romance with Vesper procedes a bit too easily too, I thought. There weren’t enough scenes in the movie tracing the arc of Vesper and Bond getting to know and like each other, so the relationship felt a bit shoehorned in.
Then there’s the reveal of Vesper’s betrayal and her destruction. In the novel, she gradually falls in love with Bond and is horrified to espy her Soviet contact lurking near their seaside retreat. Realizing she cannot shake off her Soviet blackmailers and repentant over her treason, she swallows a bottle of pills and leaves behind a confessional suicide note.
In the film, the reveal is wrapped up in the bank account/password intrigue – recontextualized at the very end as Mr. White’s payoff for sparing Bond’s life – since Bond hadn’t bothered to conduct the electronic transfer of his winnings while still in the casino! This is a blunder of monumental proportions, but is necessary for the plot, since without the money being recoverable, Le Chiffre lacks much of his incentive to kidnap Vesper and torture Bond.
The film opts for an incoherent action sequence to kill off Vesper (although I liked the parallelism of Bond’s shooting an explosive tank in this scene and the early African embassy scene), although it does manage to preserve her agency in her ultimate demise (she shrinks from Bond, embracing death by drowning to her unmasking and punishment). Unfortunately, her death lacks the eloquence and expository function of her suicide in the novel, and M must explain some of what the note did. You could say that Vesper’s character was rendered mute in the end, and not just because she was drowning in Venetian canal water… the only advantage here enjoyed by the film was that her final text message enabled Bond to secure a payback scene of his own, more viscerally satisfying than reading how Moscow had controlled her every move for about seven years. Not even Bond can kill off the KGB (or SMERSH) in one fell swoop, but for Mr. White, it’s a different matter…