How do I avoid actors casting shadows?

1. Use an ultra short-throw projector. Ultra short-throw projectors allow you to project from a close distance thereby reducing the amount of space between the light source (projector) and the surface. The shorter distance allows the triangle of light to pass over the heads of the actors.
2. Keep the actors downstage from the surface. After hanging your projector near your projection surface, turn your projector on and test out your blocking. Be aware of the area that starts creating the shadows on your projection surface and then arrange your actors to avoid that area during the performance. The center of the projection surface will require the most space from your surface whereas the edges will require less distance.
3. Think outside the box. Try creatively hiding the projector on stage behind certain props and set pieces. This is always a good way to place a projector near a screen if no fly space or lighting rig is available.