Writers have an unlimited amount of space when it comes to descriptions, lore, and backstory, but movie adaptations always have to trim the extra information. With a complex series of novels like The Lord of the Rings, this is true for most of the characters, but in the case of Frodo, movie fans might actually have an advantage.
The main character in a book can develop over time, but the protagonist in a film has to be front and center from the very beginning. Tolkien always wanted Sam to be the protagonist in the book, with Frodo being so ordinary as to evoke little response from the reader at all. Meanwhile, Peter Jackson directed both Frodo and Sam as the heroes. Most of the hidden information about Frodo is in the initial chapters of The Fellowship of the Ring, the first of the Lord of the Rings trilogy, and talks about how he came to be adopted by Bilbo Baggins of Bag End.
6 He's Originally From Buckland
This is a detail that's overlooked in the movies, and it's not an essential piece of information anyway, but it's an interesting detail from Frodo's past. Frodo spent the first 12 years of his life in Brandywine Hall, and Merry and Pippin were two of his closest childhood friends. Movie fans heard Pippin tell some total strangers in the tavern that Frodo is his cousin once removed on his mother's side, and book readers know that's exactly correct.
The relationship between these three hobbits goes way back, which is why Merry and Pippin decide they can't let their friend wander through Farmer Maggot's fields with only Sam for company. Frodo knew exactly what Merry and Pippin had been up to when he met them, because they had been raiding the same fields since they were kids.
5 Bilbo Is His Cousin
Frodo refers to Bilbo as his uncle, but that's only because of the age difference. Technically, Bilbo and Frodo are actually cousins, and this is explained when he's introduced as Bilbo's heir in The Fellowship Of The Ring. In the movie, Gandalf embraces his friend Frodo when they meet and mentions his "uncle Bilbo's birthday" and the audience just takes it from there.
All the Shire Hobbits are related in some way, as virtually all of them are descended from Gerontius, the Old Took. Frodo's mother was Bilbo's first cousin, Primula Brandybuck. Her mother, Mirabella Took, was Belladonna Took's sister. When Gandalf recruits Bilbo to be part of Thorin's expedition, he invokes the name of his host's mother, the raven-haired Belladonna.
4 Frodo & Bilbo Share The Same Birthday
It's not mentioned in the movies at all, but one of the reasons Bilbo felt a bond with young Frodo was because they shared the same birthday. This was one of the excuses that Bilbo used to throw a huge party in The Fellowship of the Ring, because it was Frodo's birthday, too.
Tolkien is never explicit about Bilbo's reasons for adopting Frodo, as the young hobbit would have been just as well taken care of in Buckland having extended family there, but the shared birthday is given as one. Another reason besides the shared birthday was that Frodo was half Baggins and half Brandybuck, and Bilbo understood that, being half Took.
3 He Managed Bag End Alone For Almost Two Decades
It seems like Gandalf is gone for only a few weeks between Bilbo's birthday and returning to warn Frodo and set him on the road, but in the books, this takes a total of 17 years. The movie doesn't have time to get into it, but Frodo had an important birthday too: his "coming of age," which means he turned 33.
Frodo's milestone was forgotten in the chaos and excitement surrounding Bilbo's party and his disappearance. He also inherited Bag End at this point, with the understanding that Bilbo would likely never return. Bag End was meticulously kept and maintained for a lengthy period until Gandalf's surprise return.
2 He Learned Elvish From Bilbo...
In the movies, Frodo longs to return to Bag End and the Shire, but this sentiment isn't present in the books. The literary Frodo is a quiet, capable traveler who speaks several languages, endures a harsh journey mostly in silence, and seems at home on the open road.
Bilbo and Frodo both liked maps, old books, and languages, and studied them together. It makes perfect sense that Frodo would have recognized the script on the One Ring as "some form of Elvish" since he knew Elvish fairly well after studying it with Bilbo's guidance.
1 ...And Spoke It Often
In the books, Frodo and Sam meet a company of Elves trekking to the Gray Havens on the coast, and Frodo greets them and speaks to them in Elvish. The same thing happens in Rivendell, Lothlorien, and several times in the books, but the cinematic adaptations never have time for it.
The few times Frodo does speak Elvish in the movies, it's done for dramatic effect, not as if he's fluent in the written and spoken language. In the books, he invoked the name of a powerful Arda, one of the beings that made Middle-earth, when he faced the Nine on Weathertop. When he brandished the Light of Elendil at Shelob, he also speaks Elvish — one example that survives in the film adaptation.