important question!

i always thought that it was that one was talking to oneself and the other was to an audience but my coworker disagrees :/
 
In a monologue, the actor need not be alone, however, none of the supporting cast (in theatre or film) speaks. When the actor is alone, perhaps thinking out loud, this is a soliloquy, not a monologue.

The English major in me says that KS and minx are GENERALLY right, although the terms do overlap. See the dictionary.com definitions.