If you’ve ever swam (swum?) in the Dead Sea (I have) you already have a clue of how you die in lava. Well, few people actually swim in the Dead Sea. If any of that water gets into your eyes they will hear your screams in Jerusalem, 40 kilometers (15 miles) away. You float in the Dead Sea. That’s because the viscosity of water that salty is so heavy. Unlike fresh water, or even ordinary sea water, it won’t get out of the way to allow you to sink.
Lava is less viscous than the water in the Dead Sea. It won’t even let you sink into it enough to float. You would just lie on top of it and sizzle like bacon. Movies like Return of the King and Volcano that show a person sinking into oblivion or sort of melting away to nothing have it wrong. Falling into lava in the real world would be much worse, and death would be much slower.
Full explanation of how this works here.
Magma is what lava is called while beneath the surface of the earth. Not all lava is the same and its viscosity can vary from so thick so that it won’t flow at all to the thin stuff found in Hawaii where it flows well and fast. The best lava to fall into would that in Hawaii because it’s more likely to immerse you and kill you quickly instead of slowly turning you into burnt bacon. But only if it’s deep. Sometimes it flows across the landscape or a highway in a thin layer only a few inches thick. Don’t fall onto that stuff.
Volcanoes that explode, such as Mount St. Helens, do so because the magma is so thick it won’t flow through the fissures in the rock. Eventually the pressure build up reaches a point where the mountain explodes. In Hawaii the volcanoes seldom blow up because the magma is more viscous and easily flows out at the surface, sometimes spectacularly.
Humans must instinctively know the properties of lava. I’ve never heard of anyone committing suicide by jumping into lava, although it would work.
Here’s how a can of Chef Boy-Ar-Dee ravioli dies in molten lava: