...what really would have happened if Stempel had just said "Marty" as the answer to the question. What could they really do to him?
This isn't the mafia we are talking about here. Sure, he may not have a future in television, but a lot of people don't have a future in TV, but we still live our lives.
If Stempel had just refused to say "On The Waterfront", the whole plan would have fallen apart.
They would just lie and make him look bad just like they did when they played that fixed tape in the office.
shareYou're right: He would've never been accepted in another quiz show. But it's worse than it sounds. He desperately needed the money. This becomes more obvious after he "loses".
shareFrom his perspective, he had nothing to gain from answering correctly. If he betrays them, they would have found a way to get him off the show short after (stop feeding him answers). Therefore, he knew that his only shot at a possible future was to play ball.
Ultimately, all of the promises they made him were false, so he was damned if he did, damned if he didn't.
He was trading winning on 21 for the panel show and possible future fame. As you said, the promised they made were false, but he didn't know that at the time he took the dive.
shareBut why kill Stempel with an "easy" question?
"I will not go down in history as the greatest mass-murderer since Adolf Hitler!" - Merkin Muffley
I really think they did it to humiliate him. They didn't think much of him to begin with, he was getting difficult. They did it just to be mean.
shareI think they should have just stopped cheating and ultimately let him lose on a hard question; one he actually did not know. That's just in real life, though. I think the film is great drama the way it is.
"I will not go down in history as the greatest mass-murderer since Adolf Hitler!" - Merkin Muffley
It was a really stupid move on the producer's part. Stempel told them from the get go that he knew the answer because Marty was one of his favorite movies and he'd seen it multiple times. The producer should have known his friends and family might smell something rotten in Denmark when he got it wrong, which could possibly lead to trouble down the road.
shareIn one of the flashback episodes, one of the former contestants did exactly that: gave the right answer when he was supposed to give a wrong one. Jack Barry does a double take as he start to say "Oh sorry" and then realizes he gave the right answer.
As other posters mentioned, if someone was uncooperative, they would either have stop feeding answers or else slipped in a totally new question.
He would have won about another $10,000 and the next time he was on he would not be given the answers and his opponent would be fulled fed every answer. His questions would probably be more tricky and aimed at places the producers felt confident he was not as knowledgeable in. He would lose and be completely out. He was hoping for that panel show and money coming in from that (although it would never match what he was making on the game show). I guess he also just wanted to stay on TV. Giving the wrong answer seemed like the only shot he had at that.
shareI imagine he feared their revenge on him in the next episode of "21" and suffer from even greater humiliation. In my opinion, his fear was well-founded: they would be seething with rage and would likely refuse to provide him with the answers, ask him more difficult questions, or even reveal his answers to his opponent in advance.