I think my favorite book-to-film change is the addition of Sam saving Frodo from letting go of the cliff and falling into the lava. And the reason why is the brilliant sound design; I wanna point this out for anyone who hasn’t noticed.
When Frodo is first shown dangling, we hear the Ring calling him. An eerie, high-pitched noise, almost a whistle, begins as the Ring first lands on top of the lava, and permeates the soundtrack as Sam looks over the edge and that expression of despair crosses Frodo’s face as he realizes what he failed to do. Just then we assume Frodo’s POV. We’re hearing what he’s hearing; the Ring is beckoning him to let go and follow it.
But then, when Sam says “give me your hand!” and it cuts to another closeup of Frodo’s face, looking up at Sam, suddenly the Ring’s whistle just … stops. Abruptly. There’s another cut to the Ring as it starts to glow and melt. It is clearly still calling Frodo, but suddenly Frodo (and we) can’t hear it.
Even the music quietens and the loudest sound we hear is Sam’s voice.
Then Frodo looks down as his hand slips, and then makes eye contact with Sam again. And then the music/chorus slowly grows louder as Sam practically whispers, and yet it almost sounds like a shout, “Don’t you let go…”
Then Frodo makes his choice. He chooses Sam, and grabs his hand.
The soundtrack just by itself reveals the true power of the love these two share. The sight and sound of Sam, pleading and pleading with all the adoration in his heart, finally beats the Ring for good. Suddenly Frodo can’t hear the Ring anymore. He can only hear his Sam.
The stronger force prevails.