"You know, all movies go through a lot of drafts, and on this one, everyone just decided to make a bigger deal out of that. What I would say is, what made finessing the script harder on this one is that we were combining two genres. We were combining a Men in Black movie and a time travel movie and we had never done a time travel movie before. I can’t tell you how many times we watched Back to the Future. But the thing about time travel is you have to really set up the rules. You have to make sure the audience understands the rules. You have to make sure the characters understand the rules and you have to have the audience understand why the characters are doing the things they are doing to achieve what they need to do. And you’ve got two different Borises, you’ve got Young K and Old K. So, making these movies are hard enough and making a time travel movie is double hard. And putting them both together is quadruple hard. That’s what took a lot of drafts, because you’d think you solved a problem in the second act and then someone would wake up at 3:00 in the morning and go, “Wait a minute, we can’t kill him first, then the other one dies too.” So obviously in a perfect world you work that out [in the script] ten years ago."
January 1, 1970
https://en.wikiquote.org/wiki/Men_in_Black_3