Structure by Design or Accident? A two-part finale was always the plan. Mr. Markus said that in one draft, the Snap didn’t occur until the second film. “But what we realized is, it would feel more like a cliffhanger than we intended,” he continued, and they had always meant to make distinct movies.

Had the Snap been pushed to Part 2, “it would be a continuation of exactly what you were watching before,” he said, when, as the culmination of 10 years of Marvel Cinematic Universe movies, it should “be as big as it wants to be, and as sprawling.”

Instead, they aimed for a first movie “that went all the way to a tragic ending. And then one where mysterious things happen that I can’t tell you about.”

Mr. McFeely said that, because Part 1 ends with the Snap, “it’s really, I think, difficult to predict where we go next” in Part 2.

Writing Approach: Though Marvel didn’t mandate any story or character arcs, Mr. Markus said, “we’re never writing without knowing where the end goes.” For instance, Captain America and Black Widow mainly defend Earth in “Infinity War,” but they “have a much bigger role to play in that second film.” In Mr. McFeely’s words, “We gave ourselves license to pay off later.”