"Just a Feeling"
This scene seems to be the most controversial Frodo moment of the RotK extended edition, so let's take a look at it. On the scene menu, this is the first part of "Cross-roads of the Fallen King," but it's really a separate event.
You might say this is the latest piece of evidence in the "What did movie-Frodo know and when did he know it?" discussion. When book-Frodo first steps out of Bag End to begin his journey he doesn't expect to see it or the Shire again, and he recognizes that the quest is a one-way trip no later than the end of FotR ("It would be the death of you...") and probably sooner.
Movie-Frodo comes to both realizations more gradually. When he leaves Bag End, he expects to go to Bree and then, probably, return home. From Bree he makes the further commitment to go to Rivendell, where we have the first exchange between Frodo and Sam about going home. Then he takes on the quest--even moreso than book Frodo does, in some ways (but that's a whole different discussion).
But his realization that the quest is a one-way trip seems to come gradually, even in fits and starts. I'd consider this quite normal, given movie-Frodo's younger age, and I have no problem with it, as long as Frodo comes to this realization (1) while he still has a chance to turn back, and (2) before Sam does. Even in the book, it seems to me that one of the major purposes of Frodo being aware of the finality of his journey is to give him the freedom to throw his entire being into completing the quest, without even subconsciously holding back just in case he'll need some of himself afterward.
I do believe movie-Frodo reaches this point in plenty of time. In fact, I think the "just a feeling" scene shows that he's already reached it. The interpretation of his "Just a feeling. I don't think I'll be coming back," as meaning it's the first time the idea has occurred to him, IMHO, runs counter to everything we see of Frolijah's reactions in this scene. It also runs counter to other scenes in the movie, although, admittedly, the added scenes in the extended edition don't always fit in perfectly with the original movie.
The director's and writers' commentary on the DVD says this scene is meant to be a mirror of Sam's "If I take one more step" moment in FotR, with Sam being the one to notice that, this time, Frodo's stopped, and to go back and encourage him. I can see that externally, but I think the dynamic between Frodo and Sam is much different, as it should be two movies later!
In the cast commentary, Elijah says he likes the scene because it shows Frodo's wisdom in knowing that the journey is one-way. The dialogue itself does provide a more obvious "hook" for people who don't catch the subtleties of Elijah's acting, and who actually need to be told (as Sam does) that Frodo doesn't expect to be returning home. But the subtleties go beyond that statement. Elijah's comment seemed to indicate that he didn't want Frodo to be seen as a victim, but as someone who knows what he's doing and makes the choice to do it, rather than someone who has things "happen to him" that he can't avoid (something that would be extremely important to Tolkien, so I may be projecting it onto Elijah's remark). As in most places where we're aware of a difference in interpretation between the movie-makers and Elijah, Elijah's seems more in tune with the real Frodo - and it's Elijah's we end up seeing on the screen.
There are echoes of a lot of scenes in this one, I think. As we go along, I'll link to the ones I've already posted screencaps from, so it'll be easier to make comparisons, but there are also some I haven't screencapped yet.