The Flash took "a lot of reshaping" to make the film "shorter".
The DC movie's director Andy Muschietti had a mass of clippings to work with as he praised lead star Ezra Miller for all the extra work they put in.
Speaking to Collider, the filmmaker said of the lengthy task of editing: "The first thing is, I like to shoot a lot and I like to have options, and depending on the actors I'm working with, we have more or less material.
"In the case of my work with Ezra, we had an impossibly long list of options because Ezra is a phenomenal actor, gives you a lot, and we did a lot of improvising that led to the scenes being longer than expected and ended up with a four-hour movie, basically."
He went on: "The first part of the answer is, yeah, there was a lot of reshaping of the movie in terms of making it shorter, basically. But of course, the editing process is the place where things really take shape.
"It's one thing to have a script and be in love with a script, but then production and the execution take you to a different place where there's a lot of new things to consider, and the edit room is where it all comes to fall into place. And that requires a lot of work.
"Sometimes I wish I had a time-travelling machine to jump from day one of edit to six months of the edit to see where the movie ends and to basically not waste all of that time trying things.
"But the truth is that it's a process that you have to go through, and it's trial and error, and a lot of different versions."
Muschietti also teased that there are some "surprises" to look out for.
He said: "If you see the movie on June 16 or after you'll notice that there's a couple of surprises there that we wanted to leave for the release of the movie, just to avoid any kind of possible leaks and stuff. So, there's a couple more surprises there."
