This is the important shot! After we establish Legolas's presence, I've posted the shot of Frodo's reaction frame by frame, so no one can say I've skipped over any evidence. Frodo's reaction to seeing the Elf is completely different from his reaction to seeing any of the others. If "even" Sam knows the story of Beren and Lúthien, Frodo certainly knows that the one kind of person he should not be seeing if he's dead is an Elf! This is a complete turnaround to what he's been assuming about his own fate - not to mention that of the others. He has to process not only "I'm not dead," but also "Merry and Pippin weren't killed by the orcs?" and, hardest to fathom, "Gandalf isn't dead?!" No wonder he's speechless; he doesn't call out Legolas's name, or give him any other kind of greeting. There's more amazement than joy in his expression. In fact, for the first 7 or 8 frames all we see is amazement. Then a thought process seems to kick in and we get a mix of bewilderment and joy.
Remember when I said we'd be seeing this expression again?