Upstage

Also called: US

The part of the stage farthest from the audience. It dates from raked stages that sloped up toward the back wall.

As a verb, to upstage someone is to stand upstage of them so they must turn their back on the audience to talk to you. Avoid it unless the scene wants it. Usage differs: most guides use upstaging for stealing focus this way, but some use "upstaging yourself" for an actor who blocks their own face from the audience.