I think this is my favorite shot. (At least today it is; tomorrow it might be something different.)
At the end of "Balance of Terror" as Angela Martine stands alone and crying in the ship's chapel, it's a close-up of her looking up towads the altar.
The doors then slide open to reveal Captain Kirk:
Shatner isn't seen approaching the doors or moving in anyway. He's just standing there motionless as the door opens to reveal him. (I think this shot probably reflects Shatner's old theater roots--where the curtain would open and would reveal him already onstage, rather then having him entering from the wings.)