The most common encounters with will-o'-the-wisps occur in marshes and swampland. Travelers will see a faint but definite flame-like light in the distance. It seems to be flickering in and out of sight. Those who approach the light notice consistently that it appears to move away from them, but if they turn around and walk away, it appears to follow them. It is believed that the light wants you to follow it, but folklorists disagree as to whether it wants to help you find your way or lead you to your death.

The optimistic view is that following a will-o'-the-wisp will lead to great treasure, much like chasing a leprechaun, but seemingly easier. The more common, and seemingly more realistic take is that these are the ghosts of the dead, not allowed in either Heaven or Hell, wandering eternally and leading travelers astray to do them harm.

Several folktales support this theory, stating that the light one sees is a single piece of burning coal from Heaven or a burning ember from Hell - both consolation prizes for trying, but failing, to get in. The name jack-o'-lantern or jack-o'-the-lantern comes from this same myth, where the burning ember is placed in a turnip or pumpkin. This is where the tradition of guiding others to your home by lighting a candle inside a pumpkin on Halloween night comes from.