Who is Frodo in the movie?
Frodo Baggins is Bilbo Baggins’ cousin and lives with him in Bag End. He’s friends with Merry Brandybuck, Peregrin Took and his uncle’s gardener Samwise Gamgee.
After his uncle leaves the Shire, he inherits Bag End and a magic ring.

Who was Frodo in the book?
Frodo Baggins is Bilbo Baggins‘ cousin, and lives with him in Bag End. He becomes 33 in the year that Bilbo becomes 111 – a birthday they actually share. After Bilbo leaves Hobbiton, Frodo inherits most of his possessions, including his magic ring. He lives in peace for seventeen years, enjoying the company of his friends Merry Brandybuck, Pippin Took and Fredegar Bolger. Then Gandalf pops up and tells him the Ring is evil and must be destroyed.

Changes in Fellowship of the Ring
Introductions
In the book, Frodo is introduced to us as Bilbo’s cousin. He only comes to the foreground after Bilbo has left, though it is made clear from the beginning that the young hobbit plays an important part in his uncle’s life. His first ‘independent action’ as a character is when Tolkien describes his concern for Bilbo’s plans after his farewell-speech at the party (1.I.). Later, he’ll be in charge of distributing Bilbo’s gifts, but he doesn’t become the main character until the second chapter, The Shadow of the Past (1.II.).

In the movies however, Frodo is the first of the ‘main characters’ introduced to us: we see him sitting under a tree, reading a book. This is of course logical: ‘Lord of the Rings’ needed a Bilbo-chapter to move from ‘The Hobbit’ to the greater story of the Ring, but the movie doesn’t need this re-introduction. This however has some consequences for the character of Bilbo, who gets a lot less attention throughout the movie.

General Character Depiction
Frodo Baggins is a young hobbit at the beginning of the book, curly and well fed (but not chubby). When the story really begins, he’s fifty years old but he still looks remarkably young for his age, though he has gained quite a bit of weight. During his travels, he looses a lot of that excess baggage: Looking in a mirror he was startled to see a much thinner reflection of himself than he remembered;… (2.I.)
Elijah Wood was of course only 18 when he was cast for the role of Frodo in ‘Lord of the Rings’, and some people claim he simply looks too young to play Frodo. However, if Frodo was 33 when Bilbo leaves, he must have looked like an 18 years old human boy. Because the filmmakers don’t include the seventeen missing years, Frodo looks like he could have looked at the beginning of the book.

Tolkien describes Frodo as a generous and friendly hobbit, much like Bilbo was, with a few good friends and not caring too much of what people think of him. This is expressed in the movie in two instances: at Bilbo’s birthday party we see a side of Frodo that we don’t see much later, namely the fun-loving extravert guy that likes dancing and a good pint in time (1.02.), and of course in the Green Dragon sequence on the extended edition, where Frodo doesn’t seem to care at all about the town folk gossiping about him.

Frodo is also a bit of an intellectual: he has learned a lot from his uncle Bilbo, reads books and even speaks a fair bit of Elvish – it is even suspected (by Merry) that he goes out to talk to Elves once in a while. He is, like his uncle was, a rather unconventional Hobbit, curious and extremely intelligent.
This doesn’t come across very well in the book: because the meeting with Gildor and the Elves was left out of the journey to Bree (1.III.) we don’t hear Frodo speak Elvish, and we’re not reminded of Bilbo’s special relationship with them until we arrive in Rivendell.
Frodo doesn’t seem as knowledgeable in the movie as he was in the book, but he is still portrayed as the most intelligent of the four hobbits.

Frodo & the hobbits
Because the journey from Bag End to Bree, which takes up no less than 6 chapters in the book, was reduced to one long sequence in the movie (1.03.- 08.), there is very little time or opportunity to establish the relationships between Frodo, Merry, Pippin and Sam. This has two important consequences.

First of all, Merry and Pippin are placed at the same height as Sam: Sam is promoted from servant to friend, while Merry and Pippin are degraded from very good and loyal friends to silly young men who get themselves mixed up in something they didn’t really want to be a part of.
In the books, Merry, Pippin and Fatty Bolger have –with Sam’s help – guessed that Frodo’s scheme to move to Crickhollow is just a diversion, and that he really wants to leave the Shire and go to Rivendell. They go with him well prepared, it’s a conscious decision.
Frodo feels responsible for his friends, and guilty when their little adventure starts to run out of hand and their lives are endangered.

This leads us to another change: Frodo no longer ‘leads’ the Hobbits on their journey. Because the journey to Bree was reduced so radically, we don’t see how Frodo takes responsibility for the safety of the group, and actually leads them by plotting a course. They seem helpless without Gandalf, and the movie audience is relieved when Strider takes charge of the group, because they seem in dire need of guidance.

Hero or cry-baby?
We can also see this change when we look at Frodo’s position within the Fellowship: where in the book he was considered the ‘leader’ because he was the Ring-bearer, in the movie Gandalf and Aragorn lead the company. Frodo is only counselled once about their course, on Caradhras (2.03.), which is ironically something that doesn’t happen in the book.

This shift from Frodo the leader, to Frodo-who-needs-protection, undermines his position as the ‘hero’ of ‘Fellowship of the Ring’. This change is also influenced by some other changes or omissions:
– hiding from the wraith (1.03.): In the movie, we see Frodo panic and dive under a tree-root, where he nearly puts on the Ring (if it weren’t for good ol’ Sam, it would have been a very short movie!). In the book, the hobbits hide on the side of the road and though Frodo ís frightened and feels the pull of the Ring, he still has enough will power to resist it.
– fighting the Barrow-Wight (1.XIII.): This chapter was completely omitted, so we don’t see a heroic Frodo fight an ugly green ghost that has already captured his companions. In the end, it’s Tom Bombadil that saves them, because Frodo had the clarity of mind to call out for him.
– the confrontation with the Witch-king (1.XI.): No such stumbling and falling and being all weak before getting stabbed: our brave little hobbit puts on the Ring, picks up his sword against the ominous leader of the Nazgûl, and stabs him in the foot!
– flight to the Ford (1.12.): Not such a short flight in the book: while in the movie we only see Strider run through the forest carrying Frodo – who looks like a corpse already – it really is a fourteen days-journey from Weathertop to the Last Bridge where Glorfindel catches up with our companions. Frodo doesn’t go all limp and helpless: when the wraiths approach, Glorfindel simply puts him on Asfaloth and sends him away!
At the Ford, there is no pretty Elf-princess to save him, so Frodo pulls out his sword again and cries the legendary words: ‘By Elbereth and Lúthien the Fair, you shall have neither the Ring nor me!’ Then he swoons. So, he doesn’t really save himself, but at least he doesn’t look like a wax dummy!
– taking a stroll on Amon Hen (2.10.): Well, that’s not precisely what Frodo was doing in the book: there he has to decide their course, but he’s torn between the various options (Minas Tirith, Emyn Muil) and his own resolve to go on alone (as Galadriel made clear to him). Because the weight of the responsibility hinders him, he goes for a stroll in the woods to think everything through. There Boromir searches him out – Frodo wasn’t running away from the camp irresponsibly.
What is more, on Amon Hen Frodo has a vision on the Hill of Sight. In the movie this vision is limited to only Barad-dûr and Sauron’s evil eye. In the book, however, he sees ‘the signs of war. The Misty Mountains were crawling like anthills: Orcs were issuing out of a thousand holes. Under the boughs of Mirkwood there was deadly strife of Elves and Men and fell beasts. The land of the Beornings was aflame; a cloud was over Moria; smoke rose on the borders of Lórien.’ The vision continues like this, describing even Rohan and Gondor. When Aragorn later also tries to also sit in the High Seat, he doesn’t see anything.

Another important factor in this change is of course Frodo’s relationship with the Ring. The Ring itself becomes a real character in the movie, and seems to take the initiative far more often than it does in the book: there, Frodo often is lured by the Ring, tempted to put it on; but Tolkien doesn’t clearly say whether it’s the Ring, Frodo or maybe even Sauron who places the attraction in that moment.