In certain movies I have seen the antagonist's arms falling off, then their legs, then their bodies split in two and then their heads fall off when the camera angle has not shifted. Or the scene can be that the body is flaking away, or is like an explosion where the body itself explodes.
Like in Harry Potter Deathly Hallows Part II where Voldemort was finally killed and his body started to dismantle as shown below:
In this he has started flaking away and decomposes.
Also in this example in 100 worst ways to die, this guy went through toxic waste, he is starting to melt:
I haven't been able to actually get a picture of someone falling apart because of IT problems.
After all this, what are the film techniques used to film these scenes when the camera angle hasn't shifted at all?