Who is Bilbo Baggins in the movie?
Bilbo Baggins is a wealthy Hobbit who lives with his cousin Frodo in Bag End. He has befriended the wizard Gandalf on one of his many adventures, on which he also found a magic ring that can make its owner disappear.
On his one-hundred-and-eleventh birthday he chooses to leave the Shire, in favour of one last adventure and some rest to finish his book.

Who was Bilbo Baggins in the book?
Bilbo Baggins is wealthy but peculiar Hobbit who lives with his cousin Frodo in Bag End, on the Hill in Hobbiton.
He went on an adventure sixty years before the beginning of this story, and thus befriended many Dwarves from Erebor and also the wizard Gandalf the Grey. He brought back many treasures from this journey, including a magic ring which can make the owner disappear.
On his one-hundred-and-eleventh birthday he decides to leave the Shire on one last adventure, and to find some rest to finish his book.

Changes in Fellowship of the Ring
Bilbo is not such an important character in the movie, and he wasn’t in the books either. He gets most of his significance from ‘The Hobbit’ – and people who read the Hobbit will thus make certain links or appoint more wait to the character.

Introduction
In the books, Bilbo is the first character we meet. Tolkien gives us a description of his life and of how the other Hobbits in the Shire view him, mainly through the characters of Ted Sandyman and Hamfast Gamgee.
Bilbo is portrayed as carefree and well-off, a real English gentleman but with some eccentricities. We don’t get any hint of his being out of form or influenced by the Ring until after the party, when he’s pulled his little disappearing stunt.

In the movies however, we are introduced to Gandalf and Frodo before we see Bilbo; and we are told about him from their perspective which is of course different from that of the villagers.
In Concerning Hobbits, the alternative opening on the Extended Edition of FOTR, Frodo voices his concern for Bilbo’s state of mind. We also see Bilbo looking for his ring with some desperation, when he thinks he’s lost it.
In the theatrical cut, something similar happens when Bilbo tells Gandalf he feels ‘thin, sort of stretched, like butter scraped over too much bread’. This line comes from the book, but was moved a little, and it is used to show us even before the party that something is not well with the old Hobbit.
So the effect of the ring, or at least a sense of unease, is established early on in the movie.

A Long-expected Party
In the books, the party is in honour of both Bilbo’s and Frodo’s birthday: Frodo becomes 33 and that is the moment of coming-of-age for Hobbits. He invites 144 guests to honour both their ages (though undoubtedly some villagers showed up unexpectedly).
He is aided in his preparations by a small legion of Dwarves, who set up the party-field and the many pavilions; and by Gandalf who’s come with many types of firework.

All this information (save Gandalf’s presence) is left out of the movie, and as a matter of fact we see Hobbits prepare the party-pavilions when Gandalf rides by in his cart.
Bilbo’s actions on the party itself are not so very different from what is described in the books, and as a matter of fact his speech is almost entirely taken from the pages without too much adapting.
However, there is no such instant described where Bilbo – hiding from Lobelia or otherwise – has a little chat with Frodo. Frodo is informed about Bilbo’s plans, but doesn’t seem too worried about his uncle in the books.

Bilbo’s Departure / A Meeting Between Old Friends
As was described above, some lines from the conversation between Gandalf and Bilbo, which takes place after his unexpected disappearance, were moved to a kitchen scene which follows Gandalf’s arrival, most likely to build up tension towards the end.

When Bilbo leaves Hobbiton, he does so in the company of some hooded Dwarves, and not on his own with only a bag pack and a walking cane. He does however, sing the song ‘The Road Goes Ever On and On’ which you can hear him sing in the movie.

One thing that was completely left out of the movie, was the fact that Bilbo had left gifts behind for all his friends and relatives; he only leaves Bag End, some furniture and his precious Ring behind for Frodo. The presents didn’t make it in the movie, and so this important trait of Bilbo’s (his generosity) didn’t quite make it to the screen either.

Many Meetings
After Frodo wakes up, he’s invited to a feast in his honour, or so he’s told. There he is reunited with his uncle, who’s become a lot older in the long years that lie between his departure from Rivendell and Frodo’s arrival there (16, to be precise).
It is on this feast that Bilbo asks to see his ring one last time, which results in a rather frightening moment where the old Hobbit becomes ‘a little wrinkled creature with a hungry face and bony groping hands’ [2.I].
Bilbo then takes his leave to finish a song he was writing with ‘his friend the Dúnedan‘ (an altogether new side of Aragorn, which sadly was left out of the movie), which he later sings for those gathered, with some success to say the least.

Evidently, this scene was altered. Frodo meets Bilbo in the gardens of Rivendell, where Bilbo is reading his own book. He tells his cousin that he would have loved to go out on one last adventure, but that he’s become too old. However, Bilbo tells Frodo in the book that he made one last trip to Dale with the Dwarves that accompanied him to Rivendell.
The incident with the Ring was moved to the scene after the council, where he hands Sting to Frodo.

The Council of Elrond
In the books, Bilbo is also invited to the council to tell his part of the tale of the finding of the Ring. He even volunteers to carry it to Mount Doom himself, and – Boromir aside, who’d never seen a Hobbit before – no one laughs at this, though Elrond does decline the offer. This shows not only how important Bilbo was to the tale of the Ring, but also how well-respected he was in the household of Elrond.
A part of the tale, namely the finding of the Ring itself, was included in the movies, namely in the Prologue.

Departure Of the Fellowship
As was explained above, a part of the Many Meetings-scene was incorporated in this sequence, where Bilbo hands his trusty sword Sting to Frodo, and also gives him the mithril vest he received as a reward from the Dwarves he aided on his first quest: we also see how Bilbo wants to take a peek at the Ring, and changes into a hideous creature.
The song from the book did not make it into the movie, like most songs I must say.

When the Fellowship leaves Rivendell, a scene that did make it to the extended cut, Bilbo is not there. However, in the books, he was.

Bilbo’s Trolls
Bilbo does make one more cameo appearance, slightly emphasised on the extended edition because of Sam’s ‘Look Mr. Frodo! It’s Bilbo’s trolls!’
When the company flees from Weathertop, they make a stop in a clearing in the woods. In the background you can see three stone trolls, which are the same trolls Bilbo is telling the children about on his birthday party.
In the books this scene was longer and had Sam’s poem and some jokes from Strider about forgetting family history and living trolls in broad daylight.

Mistakes
– One of the most common complaints about Bilbo is that, when Frodo arrives in Rivendell, he’s too old. After all, in the movie there’s no more than a couple of weeks between Bilbo‘s departure and Frodo’s arrival in Rivendell, and in that short period of time Bilbo seems to have aged very rapidly, as can be seen here.

Bookie Details
– Bilbo’s ‘red vest with brass buttons’, which you can see here, comes straight from ‘The Hobbit’.

– ‘There and Back Again. A Hobbit’s Tale by Bilbo Baggins’ is the complete title of ‘The Hobbit’, and the correct title of the book Bilbo is writing. It is shown in the movie a couple of times: in Rivendell and (on the extended edition) in Bag End

– The map Gandalf is looking at in Bag End is a copy of the original map Tolkien drew to add to ‘The Hobbit’. It’s the map Bilbo and the company of Dwarves use to get to Erebor.

– Bilbo offers Gandalf some wine, saying he has ‘a few bottles of the Old Winyard left. 1296 – a very good year. Almost as old as I am!’ This is taken from another scene in the same chapter, namely when Frodo is distributing gifts to Bilbo’s friends and relatives, Tolkien writes: ‘Old Rory Brandybuck, in return for much hospitality, got a dozen bottles of Old Winyards: a strong red wine from the Southfarthing, and now quite mature, as it had been laid down by Bilbo’s father’.