If you’re going to travel back in time, author Stephen King says, preparation is everything. The further back you go, the more you have to think about. And if you’re going to try and undo a watershed event in history—the assassination of JFK, say—you had better be determined. Because the past will do its best to remain unchanged. That’s the premise of King’s latest novel, 11/22/63, which follows Jake Epping as he slips back through time to stop Lee Harvey Oswald from pulling the trigger. To get the details right, King talked to experts about the events leading up to Kennedy’s death, and he consulted with heavyweight historians like Doris Kearns Goodwin on what might have happened had JFK lived. Wired spoke with King about the mechanics of time travel, the grandfather paradox, and the scariest thing about trying to change history.

Wired: Your main character is trying to alter the past, but everything gets in his way. He gets sick, his car won’t start, he gets beaten up.

Stephen King: There’s a kind of a rule that you’d express as a ratio: The more potential a given event has to change the future, the more difficult that event would be to change. If you wanted to go back and speak to somebody on a street corner so that they were five minutes late to an appointment—that might not be too hard. But if you wanted to stop the assassination of a president, that would be really difficult. The past would try to protect itself. My hero, Jake Epping, is befriended by a short-order cook who has a kind of a time bubble in the back of his diner. When you go through it, you always come out at the same time: two minutes before noon, on a day in September 1958. Originally the cook uses it to buy meat at ’50s prices for his restaurant and bring it back through. He always has to buy the same meat because he goes into the store at the same time, every time.

Wired: Sounds simple.

King: Well, it’s a little bit more complex than these people realize. When Al the cook tells Jake about how you can go back to 1958 and walk around and do whatever you want, Jake asks, “What if you went back and killed your own grandfather?”

Wired: The grandfather paradox.

King: Right. And Al looks at him with wide eyes and says, “Why the fuck would you want to do that?” So, in a way, we bypassed that whole idea completely. But by the end of the book, they find out that what they think is basically harmless is very harmful.

Wired: Sort of a butterfly effect thing?

King: The butterfly effect has a part in it, but my thought was that every time you go back and change something, you create an alternate timeline. There are these guardians who stand watch over all the time portals, because they understand that whenever you go back, you damage the time-space continuum. At the end Jake meets one of them, who tells him, “Every time you did this, you made the situation worse. And if you continue to do it, everything collapses.” To me that’s pretty horrible.

Wired: But every time Jake goes through the portal, everything is supposedly reset to how it was before.

King: The idea of the reset was one of the more interesting things about the book to me. You can get the idea from computers, where you can delete all this material and start over again and it never even leaves a mark. You just highlight everything, bop Delete, and it’s gone.

Wired: Well, on a computer you think it’s gone, but it’s actually not.

King: It’s like in the story. They think it’s a complete reset, but the guardian tells Jake that it really isn’t. It looked that way to you, but that stuff was still there.

Wired: If you ever came across a time-travel portal, would you go through it?

King: I guess the urge would be there, but no, I don’t think I would. I’d be afraid that the past really was a house of cards and that I might knock it down. I’d be scared.

Wired: And the further back you go, the more immediately dangerous it becomes—the more likely you are to be hounded by the villagers.

King: It’s another ratio: The further back you go, the more precautions you have to take. It would go right to the language—you’d have to be careful about the way you speak; the accents would be different. If you were to return to, say, 1858, you’d really have to prepare ahead of time.

Wired: How closely do you think people will analyze your time-travel mechanics?

King: As The Dark Tower series was concluding, physics grad students were theorizing online about wormholes and equations and all that. Plenty of buffs, some of whom read this magazine, will say, you screwed this up, you screwed that up. People make a hobby of that kind of thing.

Wired: Do you think you’ll work with time travel again?

King: No, this is it. Absolutely not. No, that’s done. It’s like Apollo Creed says, “Ain’t gonna be no rematch.”