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...