How did it happen in the movie?
After the Nazgul killed their blankets, the Hobbits follow Strider to Rivendell. Merry isn’t quite sure about Strider, until he hits Pippin in the head with an apple. After wading through some swamps and a good night’s rest, they arrive at Weathertop: a crown-shaped ruin that used to be a watchtower. Strider suggests they stay the night there, and when he goes scouting he gives all the Hobbits a small sword.
Later that evening, Frodo wakes up from the sound of talking. Turns out his Hobbit-friends are cooking some food on a little fire. He tries to put it out, but it’s too late: the Ringwraiths are already closing in on the ruin.
What follows as a feeble attempt at a fight, in which Frodo stumbles, falls, puts on the Ring and gets stabbed. Enter Strider with a torch, who sets all the walking blankets on fire and saves the day. But Frodo’s wound is fatal, and Strider decides to leave for Rivendell in the hopes to get their in time.

How did it happen in the book?
1.XI. A Knife in the Dark is a lengthy chapter, and – with the added scenes of the EE in mind – so is the Weathertop-sequence. So let’s look at the different scenes a little closer.

The first half of the sequence is taken up by the journey to Weathertop. It is depicted in a series of smaller scenes, all but one taken straight from the book:
– In 1.X. Strider Frodo assesses Strider’s trustworthiness by judging his appearance. This conversation, which revolves around the idea that ‘spies of the enemy would look fairer but feel fouler’, was added in the movie as a walking scene on the extended edition: Strider is leading the Hobbits, and behind his back they are talking about him, but Frodo deems him worthy of their trust.
– During their walk there’s a small interlude where Pippin asks if they can have a second breakfast, and Strider, completely baffled, gives them a couple of apples. This scene, the only one that made the theatrical cut, was not in the book. Also, Tolkien doesn’t mention snow since it’s only September.
– Two other scenes that made it to the extended version are direct scenes from the book. On their journey, Strider decides to cut their route short and avoid the Road by going through the Midgewater Marshes where the Hobbits are attacked by a cloud of midges and nearly driven insane by ‘neek-breekers’.
Later, when they are camping on Weathertop, Strider sings the Lay of Beren and Lúthien. This also made the movie, where we see how – after a meal Strider provided – Frodo hears Strider hum a song and asks what it’s about. In the book, Strider sings a large part of the lay and explains what’s it about – but he doesn’t reveal the parallel with his own life.

On Weathertop, Strider urges the Hobbits to make a fire: Fire is our friend in the wilderness. This contrasts Frodo’s panic when he sees his friends made a little fire to cook some food on.
The actual fight is quite different too: Strider doesn’t leave the Hobbits behind but tells them to take some burning faggots when it becomes clear the Nazgul are closing in on them; nor does he provide them with swords (they already got some from Tom Bombadill after the incident with the Barrow-wight).
When the Nazgul finally attack, Frodo slips the Ring onto his finger and attacks the Witch-king before he is stabbed himself.
The small scene where Strider inspects the wound, and the Morgul-blade turns to ashes in his hands, are described in 1.XII. Flight to the Ford.

A lot of material was cut out of the film:
– The chapter begins with a small reference to what happens in the house in Crickhollow, where Fatty Bolger is confronted with a Nazgul at the doorstep. He sounds the alarm and gets half the Shire in an uproar (our hero!).
– The story of Bill the Pony is not told, he just sort of appears when the Hobbits are following Strider ‘into the wild’. The first shot of Bill is this one. In the book, the horses of all the guests of the Prancing Pony Inn have been stolen, probably by Bill Ferny and his associates. Because the Hobbits can’t travel without their possessions, they buy an old worn pony off Bill and Sam immediately takes a liking to the animal.
When the company leaves Bree, Ferny taunts and insults Strider and is rewarded with an apple on the head – a terribly good throw by Samwise.
– Strider makes for Weathertop in the hope to catch up with Gandalf there. During their journey they see a flash of lightening jumping from the hilltop into the sky. Later they discover that Gandalf was indeed on Weathertop: he left a rune there, and their are marks of fire.
– When they arrive at the hill-site, Strider tells a little about Amon Sûl and the Men who built it. At this, Sam starts to sing a few verses of the The Fall of Gil-galad.

Mistakes
– Bill the Pony keeps appearing and disappearing. This is his first shot: how did he get there?
– It’s snowing. In September?
– Rangers are known to be practical folk, but carrying four Hobbit-sized swords on you ‘just in case’ doesn’t strike me as a very sane thing to do. So where do these swords come from?
– There’s been some complaints about the presence of corn and tomatoes in the movie: according to a lot of bookies, they can’t appear in Middle-earth since this represents the Old Continent in prehistoric times and both corn and tomatoes were imported from the American continent during the 16th century. ‘Corn’ is actually an English word for wheat, and doesn’t signify maize at all.
However, those who are being picky about these two references to the old world can explain to me where ‘pipeweed’ and ‘potatoes’ come from, if not from the New World?
– The Nazgul fear fire. Strider even advises the Hobbits to make a small fire when they arrive on Weathertop, because ‘fire is our friend in the wilderness’. So none of that: ‘Put it our!’ panic in the book.

Borrowed Lines
– The conversation between Frodo and Sam about Strider, when the Hobbits have only just left Bree, is based entirely on a part of a conversation in the Prancing Pony Inn. Merry’s question (How do we know this ‘Strider’ is a friend of Gandalf?) was originally posed by Sam: ‘How do we know that you are the Strider that Gandalf speaks about?’
Frodo replies to this with ‘I think a servant of the Enemy would look fairer … and feel fouler’. In the book he says the following lines to Strider: ‘You have frightened me several times tonight, but never in the way that servants of the Enemy would, or so I imagine. I think one of his spies would – well, seem fairer and feel fouler, if you understand.’ [1.X.S]
– Merry’s ‘What do they eat when they can’t get hobbit?!’ was originally said by Sam: ‘What do they live on when they can’t get hobbit?’ [1.XI.KitD]
– Glorfindel makes a little cameo when Aragorn says (about Frodo’s wound) ‘This is beyond my skill to heal’. This line echoes the Elf Lord’s ‘Alas! the wounds of this weapon are beyond my skill to heal.’ [1.XII. FtF]

Bookie Details
– In some scenes you can clearly see the Ring of Barahir on Aragorn’s hand. This Ring is an ancestral token of his rightful claim, however, in the books he gave it to Arwen when they plight their troth on Cerin Amroth. So this bookie detail is actually… a mistake.
– The idea of having Strider provide the Hobbits with food by letting him catch a deer, was actually Viggo Mortensen’s idea. He had this shot in mind when he first read the book, and thought it would express well who Aragorn is to the Hobbits.
– The Morgul-blade that simply vanishes is straight from the book.