How did it happen in the movie?
The scene opens with Sam, followed by Frodo and Gollum, scaling a near-vertical, jagged black mountain; they reach the top. From behind large boulders they look down and see the Black Gate of Mordor. Gollum points out that Frodo had told him to show the way into Mordor; Gollum compliments himself saying “…so good Sméagol does…” and smiles.
Looking down the hobbits and Gollum see a number of battle-ready orcs casually talking with each other on the rampart. Another is walking across it. An army is marching toward the gate. Shouts are heard and a horn is blown: the gate opens. Two great, heavily-chained mountain trolls strain to open the massive black gate.
Sam climbs up a bit to get a better look. He lays across a huge rock, which unexpectedly gives way sending him tumbling down the mountain. Anxiously Frodo goes after him.
Frodo leaves a trail of falling rocks and dust behind as he slides down the slope. Two enemy soldiers look up and see the dust. Sam is now buried up to his chest in rocks and Frodo makes a frantic attempt to pull him out. Two enemy soldiers decide to investigate and close in on Frodo and Sam. Frodo quickly covers both himself and Sam with his elven cloak. The elven cloak envelops them and takes the shape and hue of a large boulder. The hobbits escape detection. After a few tense moments the soldiers rejoin their troops.
Frodo says to Sam, “I do not ask you to come with me Sam.” And with that Frodo yells, “Now.” and the hobbits make a move toward the gate fearing it will soon close. Gollum catches them by their cloaks and pulls them back. He says, “No, no master, they catch you! Don’t take it to him. He wants the Precious. Always he’s looking for it, and the Precious is wanting to go back to him. But we mustn’t let him have it.”
Frodo once again attempts to make a run for the gate. Gollum pulls him back. Gollum tells the hobbits that there is another way into Mordor. Sam angrily rebukes him and asks why he hasn’t spoken of it before. Gollum replies that his master had not asked him. Sam remarks that Gollum is up to something and pushes him roughly. Frodo asks Gollum about the other way. Gollum slowly tells Frodo of a pass, some stairs, and then hesitating, mentions a tunnel. Gollum paws at Frodo’s arm and then gently puts his head against Frodo’s arm. Frodo mentions to Sam, He’s led us this far”. Sam pleadingly says, “Mister Frodo, no.” Frodo then says, “He’s been true to his word.” Sam beseeches Frodo again saying, “No.” Frodo turns away from Sam and tells Sméagol to lead the way. Gollum again compliments himself saying, “Good Sméagol. Always helps.”
The last of the enemy soldiers enter Mordor and the Black Gate clangs shut.

How did it happen in the book?
Before dawn Frodo, Sam and Gollum reach Mordor. They see in the west the Ephel Dúath, the Mountains of Shadow and in the north the Ered Lithui, the Ash Mountains. Between them is the Cirith Gorgor, the Haunted Pass, the entrance to Mordor. On either side of the pass is a tall, black, watchtower. These are known as the Teeth of Mordor: nothing gets by them. A fortified wall of stone is built across the pass and in this wall is a single iron gate. Sentinels are upon the wall’s battlement at all times. On both sides, beneath the hills, Orcs wait in many caves and holes to be called to duty. The black gate is called the Morannon. Only those who are summoned by Sauron or those who know the passwords can open the gate.
In despair, Frodo and Sam, hiding in a rocky hollow, observe the guards. They are about a furlong (220 yards or 1/8 mile) directly across from the top of the nearest tower. Day arrives. Trumpets, horns and drums herald the changing of the guard.

Sam mentions that it looks like they won’t get much further; he wonders what his Gaffer would think if he could see him now. (Note: Throughout this chapter Gollum refers to himself almost exclusively as Sméagol.) Gollum reveals that he knew all along that the hobbits could not get through the Black Gate. Upset, Sam asks Gollum why he brought them this way. Gollum replies that Frodo told him to take them to the gate and he has done it. Frodo declares that he intends to enter Mordor through the gate, as he knows no other way; he does not expect anyone to come with him. Gollum, pleads with Frodo not to take the Precious to Sauron. He says that Sauron will eat them and all the world. He begs Frodo to keep the Ring or to go away and, “give it back to little Sméagol.” Gollum promises to keep the Ring safe and says he will do good things with it especially for hobbits.

Sam plans to stay with Frodo to the bitter end. He hopes that now they’ll be rid of Gollum. But Gollum has other plans. He tells Frodo that there is another way. Frodo states that Gollum has not spoken of it before. Gollum replies that Frodo did not ask and that he, Gollum, has done as he promised. He says that now that he realizes Frodo intends to take the Ring straight to the Black Hand he must save them both.
Sam is doubtful of Gollum’s claim. He recalls Gollum’s debate druring the passage of the Marshes. He wonders if Sméagol and Gollum (Sam prefers the names Slinker and Stinker) have formed some kind of an agreement. Sam thinks that neither wants Sauron to get the Ring nor Frodo captured. They want the Precious within reach so that given an opportunity it can be grabbed. Sam suspects that Stinker either is or was under orders from Sauron. In Sam’s opinion Frodo is wise but soft-hearted.

Frodo addresses Gollum telling him that so far he has kept his promise. Frodo says that he will trust Gollum again but tells him that he is in great danger. He reminds Gollum of his oath and tells Gollum that he showed his true self by asking for the Ring back. He firmly informs Gollum that he will never get the Ring back. He threatens Gollum saying, “If I, wearing it, were to command you, you would obey, even if it were to leap from a precipice or to cast yourself into the fire. And such would be my command.” Sam approves of Frodo’s words. He thinks that Frodo, in his kindness, may be blind to what is really going on. He thinks that Gollum may think the same way and is trying to exploit Frodo’s kindness.

Gollum now grovels on the ground and is terrified. Frodo patiently questions Gollum about the other way. In time Gollum regains his composure. Gollum shares his knowledge of the roads that turn west and south. He then tells the hobbits about the road that turns east. He tells them of a winding, steep road and the old fortress, Minas Ithil. He describes it as a terrifying place. Gollum tells the hobbits that they must go there to enter Mordor. He mentions a dark pass and Gorgoroth. Sam asks Gollum if the road is guarded and if the tower is empty. Gollum responds saying that there are dreadful things living there: the Silent Watchers.
Gollum also shares his considerable knowledge of Sauron’s conquests. He understands that the Eye cannot see everything at all times and must focus its’ attention strategically. Gollum tells the hobbits of his travels and his encounters with other peoples. Sam becomes increasingly suspicious of Gollum’s intentions.

Gollum says, “He found it. He knows it.” He tells Frodo of a long, narrow stair, more stairs and a dark tunnel from which he, Sméagol, came out of Mordor years before. Sam asks Gollum if the path is guarded. He believes that he sees “a green gleam in Gollum’s eye.” Frodo firmly asks Gollum if he escaped out of the darkness or, as Aragorn suggested, if he was permitted to leave. Gollum reacts strongly to Aragorn’s name and his suggestion. He says that he was instructed to look for the Precious but he did escape. He has been searching for the Precious for himself and not for the Black One. Frodo astutely surmises that perhaps Gollum is telling some truths because he says, “I”. Gollum’s use of “I” seems to indicate to Frodo that Gollum’s innate inclination to truth and sincerity are currently in control. Frodo realizes that the Enemy may have allowed Gollum to ‘escape’ and may know of Gollum’s activities. Frodo senses that Gollum is holding something back. He asks twice more, “Is it not guarded?” Gollum replies that perhaps it is and says there are no safe places and that there is no other way. He does not mention the name of the place where he intends to take the hobbits.

The day passes slowly. Frodo spends time alone deep in thought about his fate and the choice before him. Sam believes that he sees four bird-like creatures; Frodo, Sam, and Gollum all become frightened. The winged creatures fly to Mordor. Sam wonders if the Black Riders can see them. Frodo answers that the Black Riders might not be able to see them but their fowl most likely can. He feels that they are looking for something.
Just as Frodo is about to make his decision about which way to take to Mordor the trio hear singing and shouting. The hobbits draw their swords. Gollum peers over the hollow. The voices fade and a horn is blown. Gollum describes the look and attire of the men going into Mordor to the hobbits.
Sam asks if the men have any oliphaunts with them. Standing up, Sam recites a Shire poem describing oliphaunts. Sam says that he has heard tales of the Swertings, a name given to the men of the Sunlands by the hobbits. The Swertings ride enormous oliphaunts. Gollum says that there were no oliphaunts and that he has never heard of them. He says that Sméagol wants to move on and hide and asks if Frodo will come with him.

The trio rests for the remainder of the day until the sun (Yellow Face as Gollum calls it) goes down before continuing on their journey.

Changes at a glace:
In the movie:
– There is not the character and relationship development presented in the book. We do not learn of Gollum’s history and knowledge. Nor do we get much insight into Sam’s or Frodo’s thinking.
– Gollum does not ask Frodo to give the Precious back to him.
– Frodo does not threaten and intimidate Gollum.
– The Easterlings bear little resemblance to Gollum’s description of them.
– The Nazgûl do not fly overhead and frighten the trio.
– Sam does not speak of oliphaunts nor does he recite a poem about them.
– The trio leaves in the daylight. In the book Gollum still does not want to travel when “Yellow Face” is out.

In the book:
– The Black Gate does not open.
– There are no mountain trolls on the battlement to open the doors of The Black Gate.
– Sam does not fall down a rocky hill and Frodo does not follow him. In turn, the Easterlings do not see rocks and dust falling and do not become suspicious. Furthermore they do not move out of line to check things out, and the hobbits do not use their elven cloaks to disguise themselves.
– Frodo and Sam do not attempt to get to the gate before it closes.
– The Black Gate does not clang shut.
– Sam, Frodo and Gollum do not leave right away. They wait until nightfall.

What the directors and writers say:
– Co-writer Philippa Boyens mentions that originally there was to be only one gate but due to a typo there ended up being two.
– Peter Jackson says that The Black Gate itself was inspired by an Alan Lee painting, as were many of the sequences in Lord of the Rings. He particularly loved the “pure fantasy” of the mountain trolls and the creaking and grinding of the gate opening. Philippa mentions that they thought that they should put someone down there close to the gate. Peter said that he then wondered what would happen if enemy soldiers nearly catch the hobbits. Peter says that the use of the elven cloak was a way of “tipping our hats” to the elven cloaks in the book.
– Philippa mentions that the last sequence when Frodo listens to Gollum instead of Sam shows a significant moment in the characters’ relationship development.

Bookie Details:
– After Sam falls down the hillside, Frodo tries to pull him out. To shield them from the Easterlings, who’s attention was drawn by the noise and the dust, Frodo pulls his cloak over the both of them, and the Easterlings see a grey rock in stead of two hobbits hiding under a grey cloak. This is (the only) reference to the actual purpose of the elvish cloaks, which is to help them blend into the background. However, this part is not in the book.

Sources
The Two Towers, Special Extended Version
The Lord of the Rings, Edition – Published by HarperCollinsPublishers, 1991