"Gollum on the edge of the abyss was fighting like a mad thing with an unseen foe. To and fro he swayed, now so near the brink that almost he tumbled in, now dragging back, falling to the ground, rising, and falling again. And all the while he hissed but spoke no words." - Return of the King (book)
Here is Gollum fighting with an invisible Frodo. Yes, it's simulated invisibility, using the same clear stand that I used to hold Morte aloft for my Planescape: Torment customs.
"Suddenly Sam saw Gollum's long hands draw upwards to his mouth; his white fangs gleamed, and then snapped as they bit. Frodo gave a cry, and there he was, fallen upon his knees at the chasm's edge. But Gollum, dancing like a mad thing, held aloft the ring, a finger still thrust within its circle. It shone now as if verily it was wrought of living fire." - Return of the King (book)
Okay, so Gollum wasn't supposed to bite of Frodo's entire HAND, but since LEGO people don't actually have fingers, it was the best I could do.
Gollum was never actually supposed to wrestle with Frodo upon the brink In the book, he just dances for joy and then slips and falls. Still, it is more dramatic in the film, really, so I can understand why they did it. This is one awful picture, isn't it?
"There was a roar and a great confusion of noise. Fires leaped up and licked the roof. The throbbing grew to a great tumult, and the Mountain shook. Sam ran to Frodo and picked him up and carried him out to the door. And there upon the dark threshold of Sammath Naur, high above the plains of Mordor, such wonder and terror came on him that he stood still forgetting all else, and gazed as one turned to stone."
Anyway, the mountain blows its top and Frodo and Sam get away and that's the end of the movie.