The heretic was burned at the stake.
In a passive sentence, the subject of the verb is the 'sufferer' of the action - the action is performed on the subject. If the heretic was burned at the stake, then somebody burned the heretic. We clearly have a transitive verb. Indeed, if a verb is used in the passive, it must be transitive.
The definition of 'transitive verb' that says it must have a direct object holds only for the active voice.
In 'The heretic burned slowly' the verb is functioning intransitively. There is no object.
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