Refocus the key light. The key light on the actor is the one providing most of the light in the scene. If the light is coming in at too high an angle or too much from the side, you can create shadows that can obscure the face. Make the light hit from more of a straight-on angle and you can eliminate a lot of shadows.
Add a footlight. Footlights are lights that are on the edge of the stage and point upward. These can create a dramatic effect and chase away shadows on the face that are coming from other lights. This effect may be too dramatic for your scene because it can create shadows on the back wall, so use this technique sparingly.
Move the offending object. If the shadow is being caused by another lighting instrument, a low-hanging cable or a piece of scenery, then the object may be able to be adjusted so it is not in the way. Tie up loose cables, slide the light over a few inches or move the scenery a little bit if you can.
If all else fails, move your actor. If you can't refocus the light or move whatever is casting the shadow, then have the actor find the light further downstage. The director may even prefer this.