Possible spoilers of Return of the King for those who have not read the book...
When I first heard that there were people who thought that Sam was a bigger hero than Frodo, I was surprised. I recognize Sam's great heroism, but I disagreed though I couldn't then put my disagreement into words. I'd like to thank, once again, Mr. Peter Jackson for giving me a new perspective on the characters.
Yes, Frodo becomes at the end a thrall of the Ring. The movie version (yes, I know there are many differences from the book) very strongly shows that the Ring affects everyone. The first indication of Frodo's addiction (again using the filmmaker's view) is his upset when Gandalf tosses the envelope holding the Ring into the fire. That is a tiny reaction, soon afterwards Frodo tries -hard- to give the Ring to Gandalf, so obviously it hasn't gotten a good hold yet.
At Rivendell, there is the scene between Frodo and Bilbo where Bilbo sees the Ring and feels its pull. I originally did not like the sequence where the monster face was briefly morphed in over Ian Holm's, I thought the actor didn't need the help of the special effects. I've recently rewatched that scene and feel differently. The point isn't how the Ring affects Bilbo, it's how Bilbo's transformation affects Frodo. The scene is very similar to the one in the book. But watching Frodo's reaction, you feel something new. Though I don't think at this point Frodo realizes it yet, he is /looking at his future/ in the face of someone he loves. Now he owns the Ring and its taint is working in him. The process seems to work faster in Frodo in the movie, but I think it makes sense. The wound from the Morgul knife surely weakened his inner core of resistance, took away some of that sturdy hobbit rootedness in the earth. He has come close to being a wraith, he has walked in the wraith world.
Sometimes I have skipped reading the portion of the Two Towers that concerns Sam and Frodo's journey. It wasn't as much fun to read. I have a new appreciation after watching the film version of the Two Towers. The film makes more sense of Frodo's compassion for Gollum. After seeing Bilbo's transformation in Rivendell, he can easily believe now that Gollum was once a hobbit. And he can imagine all too well that this may happen to him. Gollum is a vision of Frodo's future, but Frodo cannot simply give up the Ring. It must be destroyed, but now he realizes that even if he succeeds in his quest he will not escape the Ring's power. It is already working in him. This is what Galadriel points out to Elrond.
Remember, those of you who know the book, how it ends. How while the other hobbits are celebrated as heroes, the Shirefolk do not understand the price Frodo paid to bear the Ring to its destruction. Though he is still alive, he has lost his life, lost his identity as a hobbit, lost the Shire and all he loved. I don't believe that the film has altered the story in this aspect. It lets us see into Frodo, through Elijah Wood's acting. Frodo must travel under the constant torment of the Ring knowing he is making the ultimate sacrifice. Worse, he must bring Sam with him because he lacks the strength to do it alone. He lacks the strength, but not the will. This is the reason for Sam's speech at the end of the Two Towers movie. Besides being his friend, Sam is the symbol of everything Frodo loves and knows he will lose. And though Sam knows Frodo very well, this is the one thing he can't understand (or refuses to understand) that Frodo cannot any longer bear to live in Middle Earth. As brave as Sam is, he is never called upon to shoulder Frodo's burden. Maybe he would have done better as the Ringbearer, but it was Frodo who had to do it; Frodo who bears the full torment of the Ring; and Frodo who pays with his life.
Finally, there is the point of Frodo refusing to give up the Ring at the very end. I don't know how they'll work this in the movie. They've foreshadowed it in the Two Towers by showing Frodo nearly giving up the Ring to the Lord of the Nazgul. He is clearly resisting, but the Ring, in the presence of the Nazgul, is overriding Frodo's will. Sam steps in to stop him, as Gollum is destined to do at the Cracks of Doom. And there, as Tolkien said specifically, is the heart of Sauron's power in which all other powers are subordinate. I read this as Frodo having no choice at this point. Tom Shippey, author of the excellent 'Tolkien: Author of the Century', points out that Frodo says 'I do not choose to do what I came to do,' and that Tolkien being a philologist was keenly aware of word choice. Frodo does NOT choose; it is Sauron's will being done. Consider his apparent choice, to claim mastery of the Ring. Brilliant, no? At the last minute, on Sauron's doorstep? Suppose he'd tossed Sam and Gollum into the volcano, there is no way he would have the power to oppose Sauron. And he knows that Sauron is aware of him and knows exactly where Frodo and the Ring are. The only person who could possibly benefit from Frodo's claiming the Ring is Sauron. I think it's pretty clear that Frodo is off his nut and cannot be held responsible for being unable to throw the Ring into the volcano.
That all said, I really am looking forward to Return of the King and am tormented by the months I'll have to wait. Argh.