Maybe it was the stage where her star would have shown the brightest. I say that because as it was shot on video tape, this episode has a theater feel to it and Inger Stevens is the "grande dame" of this short play.
As a former actor/director myself (not on any big scale or even small), I can tell that this young lady clearly did her homework on this part and she took advantage of the wonderful words and character she was given to create, I suspect, in a very short amount of time. Each motivation was strongly chosen and came from deep within her. I like Inger Stevens because she's raw and she's real. I don't see that too much with today's stars (as opposed to actors).
In defense of the six episodes that were shot on video. I like them and I wish they did more (but certainly not all)! I like to intimate, live TV/theater, daytime soap opera feel that black and white video tape gives. For this particular episode, it was totally appropriate to shoot this on video. I think it made it even creepier. Twilight Zone was partially about experimentation with this new medium of TV and I, for one, am glad they did.