The human body does sometimes move during cremation but they do not sit up.
Bodies sometimes go into what is called a pugilistic stance due to the high heat of the cremation chamber which means the knees, elbows, and fists clench due to dehydration because of the extreme heat.
The pugilistic stance can make the body appear stiffer and shorter but it does not cause the body to sit up during cremation.
The body does not feel pain when being cremated.
Dead bodies cannot feel pain as the brain is shut down and cannot send signals to the body through nerves.
So once you're dead you cannot hear, see or feel any pain or have any emotions etc.
The body does not know anything about what is happening and does not feel any fear or pain when they get cremated.
The body does not feel pain during cremation.
When someone dies the person does not feel any pain or have any senses anymore as the brain has completely shut down and every organ has also shut down.
So the person being cremated will not know they are being put into the cremation oven and being cremated.
If the person feels pain then they are alive and not dead.
But if the person is indeed really dead then the person will not feel or experience any pain or discomfort from being cremated.
Cremation although it seems crude is actually a better option than burying a body and it's more environmentally friendly as well.
The body is burned up and the bones are ground up into powder which takes less space than a body being in the ground.
Also no coffin is needed and no concrete for burial vaults are needed either.
Cremation can cost thousands of dollars less than traditional burial of the body.