Front versus Rear Projection

Front Projection

Front projection is the most commonly used projection technique. It merely involves projecting onto a surface from in front of the projection surface. In theatre, this leaves a number of options for where to place the projector:

From the booth: This placement involves using a long throw projector from the booth or the back of the house. There will be shadows, regardless of blocking.
From a Light bar: By hanging an ultra-short throw projector from a light bar, above the actors, you're able to project over their heads, eliminating most shadows.
On the Stage: This placement involves hiding the projector upstage behind some set dressing. This can work if you have an ultra-short throw projector, but it's easier for an actor to accidentally block the light if they move too far back. Hanging the projector from a light bar or the grid is much more ideal.

Rear Projection

Rear projection isn't as common as front projection, and it's mainly because it requires a specific stage setup that many clients don't have. Ultimately, if using rear projection you can place the projector in one of two spots:

On the ground behind the surface: This placement have the projector sitting on a table or the ground behind the surface. This can prove troublesome for some clients because NOBODY can walk through the beam of light backstage without casting shadows. This can create anywhere from 10 to 20ft of unusable space on the stage.
Hanging behind the surface: This placement alleviates some of the previous problems, but not many clients find themselves with a light bar behind the surface.

Keep in mind that for rear projection, you will also need a slightly translucent material scrim to project through. This produces a visible light source and can be more expensive than a traditional cyc or skrim.