Simplest solution for a manual exposure at a constant setting is to wait until the light level goes down outside then use lights inside the room to light the actor. If you have to film in bright daylight, a solution (although costly) is to gel the windows with Neutral Density gels which you can buy on a roll. If you have tungsten lights you can buy combo ND/colour correcting gels to cover the windows. Its perfectly acceptable though to allow the outside to be overexposed by a couple of stops