Burning at the stake was a very common way to execute blasphemers, thieves and witches. It was used throughout the Middle Ages and beyond.
If the fire was big enough, death occurred first by asphyxia rather than damage done by the flames. However, this was a known fact and the victims were usually burned in a smaller fire so they would "suffer until the end". When the fire was small, death occurred because of loss of blood or a heatstroke which could take even hours.
When the victim was hated by the population if he, for example, raped a woman, the general populace often congregated around the stake to see the victim die. The smell was terrible and lasted for many hours or even days after his death.