Erik Klemetti points out something obvious but frequently ignored by movies: you won't sink if you fall into lava.
Molten lava is nothing like water. Sure, everyone thinks that liquid rock (magma) is going to behave like any other liquid (e.g., water), but there are some key physical properties that tell us it just isn't the case. Let's compare!
- Water has a density of 1000 kg/m3 and a viscosity of 0.00089 Pa*s.
- Lava has a density of 3100 kg/m3 and a viscosity of 100-1000 Pa*s.
Pa*s is the SI unit for viscosity -- some people might be familiar with other viscosity measures like poise. Viscosity is, more or less, the resistance to flow, so if you throw something in a liquid, a low viscosity liquid (like water) will "get out the way" and you'll sink faster relative to a high viscosity liquid (like cold corn syrup). The density of the liquid will also play a role in how quickly you might sink based on your own density. So, when we're looking at water versus lava, lava is ~3.1 times the density and between ~100,000 to 1,100,000 times the viscosity. They are very different!
You could probably walk on lava, except for the horrible burning. Even still, you could amble on your ever-shrinking legs.