You're sitting by the fire one night, reading your book, when suddenly there's a gust of wind. The light flickers and you see your shadow move... except you're not moving. You get up and see that there are actually two shadows — one is yours and one, well, that one's moving independently of you. How (creepily) amazing!
This is the Living Shadow. Sometimes it's evil, sometimes it's good, sometimes it's merely mischievous. It may be a ghost, a person with the power to become a shadow, a shadow magically animated and separated from its person (who often Casts No Shadow), an alien that only appears to be a shadow, or something much, much worse. Regularly, they're used as Nightmare Fuel.
Voluntarily separating yourself from your shadow is one of the more dangerous things you can do. It usually has the obvious corollary that you Cast No Shadow but that's merely the obvious.
Not to Be Confused with a Shadow Archetype, which isn't usually a Living Shadow, or with Fighting a Shadow, or Loving a Shadow (which may happen with a Living Shadow, but is not necessary). Also not to be confused with Casting a Shadow, though it's not uncommon for the two to overlap.