The first couple of Bond movies were sort of based on the books.
By the time Roger Moore took over the role, all they had in common were the titles.
Hell, even the movies that were based on books sometimes barely resembled the books. They hodgepodged those books like crazy. Hell, scenes were deleted entirely only to be put into another movie (even ones that did not even used one of Fleming's titles like the shark scene in Licensed to Kill).
Someone mentioned Troy, and that one really burned my ass at the time. The movie was not bad, per se, but when you are basing something off of a couple of the true classics of literature (The Illiad and Aeneid), you should not use too much artistic license. Tradition is against you there. Let us compare some of the differences:
Duration of the Trojan WarClassic timeframe - 10 years
Movie timeframe - less than a week
Reason the invading army was so largeClassic reason - It is a little convoluted, but it stemmed from all the powerful kings of the world vying for Helen's hand in marriage. As it was assumed that those that did not land her would be resentful and try exactly what Paris pulled off, Odysseus proposed that all make a sacred oath that they would all defend the honor of whomever Helen chose as her husband should the need arise. The need indeed did arise.
Movie reason - Agamemnon threatened and bartered all the other kings just to conquer Troy using his brother's marital issues as an excuse.
Reason why Patroclus posed as AchillesClassic reason - Despite being a superior warrior (being a demigod and all), Achilles was never too committed to the cause (he had actually tried to dodge Menelaus's call by cross-dressing at his mother's behest). He grew tired of 10 years of fighting and more or less said "F*** this", especially as he saw Hector's tactics were tearing the Greeks a new one and he had already fought Hector one on one and they both survived. Considering that in order for the Greeks to win, Achilles would be needed to actually kill Hector in order to get someone less capable to run the Trojan defense, he viewed his inability to kill Hector as an omen that the Greeks were destined to lose.
Movie reason - He was too busy getting captured sex slave action.
Reason that the Achilles/Hector duel was a big dealClassic reason - After 10 years of fighting, the fields of battle had established many characters as true badasses. The two badasses of all badasses were Achilles and Hector. Basically, it was every Schwarzenegger character rolled into one facing off against every Sylvester Stallone character rolled into one. Gods not only stopped everything to watch this rematch, but Athena directly interfered in the duel (in Achilles' favor).
Movie reason - More or less, it was because two of the biggest stars of the movie were fighting. They had not done much to establish Hector as a badass besides have him kill Menelaus and Ajax the Greater in battle (who actually survived in the classic telling of the story).
Reason why Achilles got so bent out of shapeClassic reason - Patroclus was Achilles' object of pederasty, which in ancient Greece was fairly commonplace. In fact, it was considered a standard relationship for men of power and influence to have. In ancient Greece, men could love women, but love between men was considered true love (and it is the source meaning of the word agape, which amuses me to no end). It is the reason that Greeks today still hear gay stereotype jokes about themselves. Also, when Hector killed Patroclus he stripped him of Achilles' armor and claimed it as his own. Considering that it was made by Achilles' mother (who was a god), this was a bit of a big deal as well.
Movie reason - I guess he liked the kid. Really, they did not go beyond that (though I guess I understand why not).
How much the gods got involvedClassic story - Boy, it is really hard to find a story that had them more involved than this one. It all started because, believe it or not, Eris did not get invited to a party of the gods. She responded by throwing some apples of discord about and three goddesses (Hera, Aphrodite, and Athena) each picked one up. The power of the apples made them each want to compete to see which was the most beautiful amongst them. As Zeus knew better than to get involved, he appointed Paris as the one who the goddesses should have judge their beauty. Being a mere mortal, he was overwhelmed by having three goddesses pose naked for him to judge which was the most beautiful. As he could not decide, they instead all offered him some fantastic bribes. He took Aphrodite's bribe (love of the most beautiful woman in the world) and named her the winner. Hera and Athena did not take too kindly to the rejection. The resulting war dragged all manner of gods into the mix as some gods favored certain heroes on both sides of the war, some hated certain heroes (particularly Hera and Athena), and some just wanted in on the fun (Ares might have fit in here).
Movie story - Yeah, no gods were in the movie.