Regarding The Walking Dead, I've noticed that even when a zombie is burnt ridiculously, it's still hissing and longing for food. Wouldn't the brain be destroyed- not just neurologically, but physically- in the kind of heat a charred body creates?
In Universe Answer:
We're probably supposed to assume that burning a zombie enough - that is, at a sufficiently high temperature for a sufficiently long period of time - will indeed kill it, but if a charred zombie is still somewhat active, the skin and flesh were burned but the brain was more or less unaffected. For example, in season 6 episode 2, a member of the Alexandria community was hit with a Molotov cocktail, and burned to death. He then reanimated, presumably because external injuries from burning gasoline are enough to mess up your skin and flesh, but the flames might go out before your brain is cooked.
It is very rare for a burn victim to die of brain damage. In most cases, death results from shock, loss of body fluids, electrolyte and salt imbalances, and so on.
Severe burns cause immediate nervous shock. The victim grows pale and is confused, anxious, and frightened by the pain and may faint. Much more dangerous is the secondary shock that comes a few hours later. Its chief features are a dramatic fall in blood pressure that leads to pallor, cold extremities, and eventual collapse. This secondary shock is precipitated by loss of fluid from the circulation, not just the fluid lost in the destroyed tissue but fluid that leaks from the damaged area that has lost its protective covering of skin.
Burns kill not just by damaging tissue but by allowing this leakage of fluid and salts. If more than a fifth of the blood volume is lost to the circulation, insufficient blood returns to the heart for it to maintain blood pressure. And the loss of salts, particularly sodium and potassium salts, not only disturbs their balance in the body but changes the osmotic balance of the blood and body fluids. The significance of these physiological changes was understood in 1905, but not until the 1930s were doctors able to correct them with transfusions of blood or plasma.
- Encyclopedia Britannica
Out of Universe Answer:
This is probably one of those times when you're supposed to employ the willful suspension of disbelief. Even if the heat wasn't intense enough to boil the brain, it would probably cook the muscles, and cooked muscles don't work anymore. The nerves that allow brain signals to move the muscles would probably be significantly damaged as well, so a thoroughly burned zombie with an intact, uncooked brain would most likely be unable to move. There are other phenomena which require us to resort to the willful suspension of disbelief:
The ease with which the skulls of zombies - and even humans - are pierced with knives. Zombie skulls are also incredibly easy to crush for some reason. When Fear the Walking Dead came out, the producers said that one of the differences we would see between the original show and the new spinoff would be the relative strength of the fresher zombies in FTWD. They specifically said that we would see a knife bouncing harmlessly off a zombie's head1, and that is precisely what happened in the second episode. However, on the original show, it isn't just old, seriously decayed zombies who can be stabbed in the brain without difficulty - knives easily penetrate the skulls of brand new zombies, and even humans who haven't turned, or even died yet. This leads me to believe that stabbing heads is always as easy or difficult as the writers need it to be.
Skin and rib cages made of tissue paper. Similar to the softness of skulls, mentioned above, it appears that people are unbelievably easy to tear apart. On many occasions, an unfortunate character is grabbed by zombies, thrown to the ground, and almost at once, the zombie(s) rip through their abdomen, possibly even their ribcage, and begin disembowling them and making garters out of their intestines. In real life, most people can't just shove their fingers through the flexible skin and subcutaneous fat, and the thick, tough layers of muscles, that protect the internal organs - much less the ribcage which evolution designed to protect the lungs and heart.
The absurd rate at which the survivors pull off head shots on moving targets, often with handguns. In the real world, head shots are very difficult to achieve consistently, especially on targets that are moving in an erratic, shambolic manner. Perhaps with years of experience under their belts, Rick's group has learned to do this better than most other people, but we saw them doing it pretty well from the very first episode of the first season. This is simply unrealistic, but we have to accept it and move on.
The fact that the survivors are still able to drive around so much. Gasoline breaks down fairly quickly when left in storage tanks. On average, after 2 years or so, it should have deteriorated so much that it can no longer serves as fuel for internal combustion engines. Diesel fuel lasts a bit longer, but still has a limited shelf life. Two years or so have passed on the show, so driving should now be pretty difficult for our survivors.
Hollywood deaths. People in The Walking Dead usually die the way they do in most movies and television shows, which is to say, not the way people usually die in the real world. When someone is stabbed in the stomach on the show, they slump over dead almost immediately. In the real world, death is usually much less merciful in such situations, and comes only after a lengthy period of agony and internal hemorrhaging.
Of course, the very existence of zombies is more implausible to most people than anything else, not to mention the fact that the zombies are all but immune to the natural processes of decay that should turn them into a mouldering pile of rotten meat within a matter of days, weeks, or months. However, as a zombie fanatic, I have taken the position that in any given piece of zombie media, the zombies themselves should be the only unrealistic aspect of the story. The only zombie media I have found that totally lives up to this standard is Max Brooks' book World War Z. The Walking Dead does better than most other zombie stories, but it occasionally falls short. We just have to accept that and focus on the positive features of the series.
On the other hand, in some cases we see burned zombies that are all but dead, virtually incapable of movement. This was the case in season three, when Milton set fire to the pit full of zombies.
These thoroughly toasted walkers were basically reduced to crispy cinders, weakly snapping their jaws. The same seems to have been true of the aforementioned Alexandrian who was hit with a Molotov cocktail. When his charred corpse reanimated, it was a pathetic wreck, laying on the ground, snarling and biting at the air. We can probably assume that, when we see more or less fully mobile burned zombies, it is primarily intended as a visual change from the typical, unburned, rotting flesh zombie we normally see.
1From an interview with Fear the Walking Dead showrunner Dave Erickson:
Erickson said one of Kirkman's notes for Fear was that the zombies be different than how they were first seen on the original show. With the early onset of the outbreak, skulls will still be hard and the point, Erickson said, is to see a character trying to stab a walker in the head and not be able to penetrate the skull. "The point was it's hard physically to kill somebody," he said.