The Hero has come upon an Innocent Bystander trapped in a Tailor-Made Prison by a wicked monster. The prisoner may plead with the hero to be set free, telling him all about the horrors the tyrant has inflicted upon him, or the prisoner may just hang in sorrowful Unwilling Suspension in his terrible bonds. Once the hero sets the prisoner free, it quickly becomes apparent that he was tricked.
The False Innocence Trick (aka Fake Good In A Can) is when a villain or monster locked away for a good reason feigns innocence or being a good guy in order to fool the hero into letting him go. Typically the villain will prey on the hero's good will and strong moral fiber while doing this by making himself and his story as sympathetic as possible. He often is also counting on the fact that the hero has come from a time/place far enough away to not know who the prisoner really is (or, if he's a local, simply hasn't heard of him).
Compare The Farmer and the Viper and Bitch in Sheep's Clothing for other cases of villains taking advantage of the hero's trusting nature.
Contrast the more common Decoy Damsel, and Disguised Hostage Gambit for when a villain makes a genuine prisoner look like a bad guy.