The person you refer to is the living-room version of the guy or gal who comments on the sporting event as it unfolds on TV in real time: a color commentator.
There's Smith on the 40-yard line. He's wide open, but . . . Oh no, he's been tackled hard by number 63, Hulking Halfback, who caught him unawares. The football falls to the ground. Incomplete pass. That's the fourth time in this game alone that Hulking Halfback has prevented a completed pass. Looks like it's time for the QB to find a new receiver, and preferably a receiver who is, wherever Halfback is not. I see a penalty flag has been dropped at the 40-yard line; Hulking Halfback has been caught--finally! His coach should take him out of the game. What poor sportsmanship!
Here's your family's color commentator, waxing eloquent during a primetime sitcom:
It's about time that nosy neighbor got put in her place. What a busybody! I'll bet she doesn't open her mouth again for the whole episode. Just because she thinks Jimmie shouldn't be allowed to go to the party, doesn't mean Jimmie's parents have to feel the same way. Geesh.
[After the first commercial:] Boy, I didn't expect the parents to let Jimmie to go to that party. He's only 16. You just know he has some mischief on his mind. He'll probably get drunk, and when he comes home at three in the morning his parents will be waiting for him in the living room, eager to give him a good reprimand.
I wonder if his younger sister will find out? Then the parents will need to teach her about the evils of teenage drinking. That'll come after the commercial.
[After the commercial:] Yep, I told ya. Here's little Jimmie coming home drunk as a skunk. Oh, I didn't expect this: here's little sis toddling down the staircase as her brother pukes on the living room floor! Ewww. Gross. Now the whole family will have to have a pow-wow. Nope. Didn't expect that! They're sending Jimmie to bed and telling his sister that Jimmie isn't feeling well. They'll probably straighten things out in the morning.
[The next morning:] I knew it. Jimmie was going to wake up with an awful hangover! It's noon, and his parents are waiting for him to come downstairs. Maybe that nosy neighbor was right: Jimmie shouldn't have been allowed to go to the party. Anyone feel like some popcorn?
And so on.