A monoact is an adapted monologue, like one that you would use to audition with. It is also like a scene, having all the elements you used for performing your scenes but with only you performing. The 'invisible' person is usually placed in the audience area. The actor, however, maintains the fourth wall. In some instances, it may be appropriate to place the 'invisible' person in the scene with you. It is always best to choose the placement that will be most advantageous to your blocking.