Introduce some elements that are definitely wrong.
Introduce some elements that are definitely wrong. At At the very start of the game, throw something in that makes it absolutely clear that this isn't exactly the 1920s California they're expecting.
When they notice a discrepancy, make it seem intentional.
When they notice a discrepancy, make it seem intentional. Let's Let's say they're talking to the mayor of San Francisco, James Rolph, and they call him by his nickname "Sunny Jim". The players know that this was a nickname he liked, but you don't know that, so you have him take offense to it.
Make up information for the characters to know, and give it to the players.
Make up information for the characters to know, and give it to the players. If If you're going to do an adventure out in the farming towns of the central valley, and one of the characters is from there, give them a map you made up yourself. You can do a bit of research to get the basics, but it's ok to make up towns or leave places out. The players already know that the world isn't quite like ours. And when you give this map to one of the players, representing their character's knowledge, now they get to be the one presenting it to the party. The fictional facts become their facts, not just your facts that the players are disagreeing with.
This way, the players know the genre, but not the story.
This way, the players know the genre, but not the story. Most Most of their general knowledge will be correct. Laser pistols don't exist. Telephones do, but they're not too common. Newspapers carry stories from across the globe. Africa is dark and mysterious. San Jose is a quiet farming town. But their specific knowledge doesn't give them any spoilers. They won't be able to predict the 1929 stock market crash, because they know that this isn't exactly the world they've read about. They can't use Wikipedia to learn things their characters wouldn't know.
