If the fire is big (for instance, when a large number of heretics were executed at the same time) the death comes from the carbon monoxide poisoning before flames engulf the body. However, if the fire is small, the convict burns slowly and dies in great pain.
According to ancient reports, Roman authorities executed many of the early Christian martyrs by burning. These reports claim that in some cases they failed to be burned, and had to be beheaded instead. However, all such ancient manuscripts were copied by Christian monks, and even Catholic sources state that many of these claims were invented (or "apocryphal").
In 1184, the Synod of Verona legislated that burning was to be the official punishment for heresy. This decree was later reaffirmed by the Fourth Council of the Lateran in 1215, the Synod of Toulouse in 1229, and numerous spiritual and secular leaders up through the 17th century.
Witch trials became increasingly popular through the 14th and 15th century in Scotland, Spain, England, Switzerland, Austria, and Germany. It is estimated that up to four million convicted witches and heretics were burned at the stake during this time.
Among the best known convicted heretics to be executed by burning were Jan Hus (1415), Joan of Arc (1431) and Giordano Bruno (1600).
Contrary to popular history, none of the executions in the Salem witch trials were carried out by burning, but rather by hanging (and in one case, by pressing under stones).
During the reign of Queen Mary in England (1553-1558), some two hundred and seventy seven people were burnt at the stake for heresy against the Catholic church and conspiracy against the Queen, including Thomas Cranmer, Hugh Latimer, and Nicholas Ridley. Between 1555-57 seventeen protestants were burnt at the stake outside of the Star Inn in the town of Lewes in Sussex. The traditional bonfire celebrations held annually in the town on 5 November commemorate the burnings as well as the Gunpowder Plot of 1605.
Also see : Witchhunt, Trial by drowning