Micheal - I think Celticruins said it well, but I have to add my own take too.
As someone who saw the films first and then read the book, I don't agree at all that there was less development in the movies - it was different, but in some cases more. Especially Aragorn.
But first
the Hobbits - All of them start off as small as their stature. Sam scared of Rosie, Frodo care free and in wonder of his uncle, Merry and Pippin complete idiots.
They take the adventure, Frodo because there is nobody else to do it, Sam because he is a good friend and Merry and Pippin because they fall into it and aren't about to let others go off and have more fun.
Over the course of the three movies we have some amazing dialogue from these characters that is lifted from the books, I think mostly, and it goes along with what we are seeing from their development.
Frodo: from wanting to kill Gullum in FOTR to defending him in ROTK - his growth in understanding the power of the seduction of evil and his own frailty in standing up to it is incredible. He was judgmental in his carefree innocence and an independent nephew of a fairly rich uncle - but he becomes compassionate through his deep understanding and sorrow and learns he too depends on others.
Elijah does this transition beautifully.
Frodo obviously has no idea what he was getting into when he volunteered, doesn't even know which way to go, doesn't know what he's giving up, and you can see that in his face. Over the course of the movies we see him slowly realize this and he goes from overwhelmed to deeply burdened.
Sam: Basically a good friend but a coward in the face of failure. He slowly over the movie starts to understand the importance, not just of what he wants to do for a friend, but the overall picture. His monologue about the past stories and heroes and how they never give up was excellent writing! Celticruins said the rest about the spider and all, but also, the way Sam goes after and wins Rosie... it's all there. He changed - he is no longer a bumbling fool, he is a confident and like his name 'wise'.
Pippin: He is much more than comedy relief in the end. He just about gets them killed in Moria by giving up where they are through his curiosity, but when his curiosity gets him eye to eye with Sauron, he does NOT giving away Frodo... Very strong mind to resist that! In the end he is willing to try to make up for losing Boromir by swearing allegiance to his father and then is able to figure out what's about to happen to Faramir and getting Gandalf to stop it while all the other men stand around and let it happen. Fool of a Took is no more by the end.
Merry: His biggest growth is in Fangorn. He is still the brains between the cousins obviously, but between tricking an Ent to comforting Eowyn and assisting in bringing down the witch king, by ROTK he is so more than the sneaky fire works stealing hobbit in FOTR.
Now,
Aragorn - when I watched the movie I had very little knowledge about who he was going to become. I think Jackson did a great job introducing him as a mysterious, dark figure who is, as Frodo says, 'not very fair'.
In the book he is described as kingly when they ran into Eomer, Eomer could see just by looking at him how majestic he was. But the movie held off on that development slightly... We do see an Aragorn who doesn't want to be king or to lead people, and doesn't seem to feel worthy of Arwen... We see that she believes in him and the others around him see something, but he doesn't believe in himself.
The change comes at Helm's Deep.
The conversation between Aragorn and Theoden was amazing... Here we see Aragorn running off at the mouth about his true feelings - the people are not prepared for this situation... too old or too young etc. And Theoden models for him what a king is - puts him in his place and from that point on we see a different Aragorn.
Something starts to come over him when he sees what a good king can do for the people, inspire them, motivate them and that it makes a difference. His mission becomes less about saving Middle Earth and more about caring about the people who need him as a good king.
I don't think he's reluctant to be king in the end at all. His speech that precedes the famous 'for Frodo' is epic. The height of his leadership, a total turn around from mocking the Hobbits for not being afraid enough of what hunts them.
I could go on... but I will only concede Legolas. He was so highly underused in the movies and in the book. He's basically the pretty face that gets to say the obvious that everyone else already knows. And he's not even that good with the bow, it turns out:
Reinventing the fastest forgotten archery. - YouTube
:lol