John Scalzi, noted Sci-Fi writer, challenges nerds to justify their whining about how this or that isn't realistic. Why do they accept some otherwise-crazy events in the stories they read but not others?
This is about "suspension of disbelief," the foundation that allows fantasy literature to work. It's a pact between author and reader -- the author asks the reader to accept something unbelievable in exchange for an interesting story. It's a kind of bargain, but the onus is entirely on the author. If the author pushes the reader too far the bargain breaks down, and the fantasy crosses the line into the absurd.
Scalzi calls this a "Flying Snowman":
When my daughter was much younger, my wife was reading to her from a picture book about a snowman who came to life and befriended a young boy, and on each page they would do a particular activity: build a snow fort, slide down a hill, enjoy a bowl of soup and so on. The last three pages had the snowman walking, then running, and then flying. At which point my wife got an unhappy look on her face and said ‘A flying snowman? That’s just ridiculous!’
To which I said: ‘So you can accept a snowman eating hot soup, but not flying?’ Because, you know, if you can accept the former (not to mention the entire initial premise of a snowman coming to life), I’m not sure how the snowman flying became qualitatively more ridiculous.
A snowman comes to life, he eats hot soup, he flys -- what finally breaks the fantasy writer's pact? The answer is simple -- what's not required for the story. Snowmen come to life all the time in fantasy, as do all manner of anthropomorphic artifacts. But why? Do they come to life to explore the boundary between living and dead? To terrify people as unstoppable foot soldiers of darkness? Or is this a study in ice-based life forms?
No, in this case the purpose is for a human child to have a fun companion. Someone to do all the things that a child would want to do in the winter. Building, sledding, eating soup. Those all make sense for that story. For an 'ice-based life form' story soup is a problem, but for the 'companion' story eating soup is OK.
But flying? No, that doesn't fit the concept as required for the story (at least as relayed here). Thus, disbelief dissolves.
This whole discussion originated with a thread about Gollum falling into lava at the end of Lord of the Rings, and how he shouldn't have sunk. Two things about that.
First, the fantastical elements that Scalzi points out as potential flying snowmen really aren't: impossibly large spiders, talking trees, rings freighted with corrupting evil, Uruks birthed from mud. These are all really the same thing. This is a world where evil exists as a material force, and all the other oddities derive from that. Evil makes spiders huge and animates the Orcs and Uruks. Its opposing force presumably makes eagles giant and motivates the Ents. But lava's just lava.
Second, everyone always gets lava wrong. Molten rock isn't an everyday occurrence, and lava lakes aren't a human-scale phenomenon. You can't walk out on to a bridge and look down into a lava lake. The first problem is the heat. If you've ever been at a glass blowing demonstration you've gotten a tiny sample of it. When the glass furnace is open you can feel the heat biting into your flesh and the air starting to dry and heat up fast. After only a few tens of seconds it starts to become unbearable. Now imagine that expanded to fill a mountain, and opened long enough that the surrounding rock is heated through and the air is as hot as it can get. If the Hobbits managed to walk into that they'd be cooked before they got to the edge.
Of course they wouldn't get very far anyway because of the air. Melting rock releases a lot of gases, none of them fit to breathe and many of them poisonous. In what is depicted as a relatively enclosed space, unless Gollum can metabolize sulfur he'd be long dead before he got anywhere near the lava.
The fact that lava can boil and flow belies the huge forces required to move it about. It's dense, viscous and very sticky. If Gollum's body were to land in it, it would most likely lie on the top or be dragged under by the force if its movement, like a bit of straw in a pasta pot. It would then be quickly consumed, like a snowflake in hot soup.