Now the filmmakers are ready to pull back the curtain on the Marvel Studios film, which has passed Titanic to become the No. 2 movie of all time. On April 30, the filmmakers spoke at a Q&A moderated by NPR pop culture correspondent Linda Holmes at the Freer Gallery in Washington, D.C. and hosted by Smithsonian Associates. Media in attendance — including The Hollywood Reporter — were not allowed to share details of that conversation until now.

Among the things they tackled were the final shot of the film, which revealed that Steve Rogers (Chris Evans) stayed in the past to be with the love of his life Peggy Carter (Hayley Atwell). Fans have debated this moment, as the Russos' first Marvel movie, 2014's Captain America: Winter Soldier established that Peggy Carter had a husband, a man whom Steve Rogers saved in World War II.

Since Steve Rogers is a noble man, would he really have gone back in time and essentially broken up a family by reuniting with Peggy? Does this mean that somehow Steve was actually Peggy's husband all along (as one theory goes)? Or did he create an alternate reality with his actions? According to the Russos, it’s the latter.

"If you went back to that timeline, between the point where Steve went into the ice [in Captain America: The First Avenger] yet before Peggy met her husband, Peggy was available," Anthony Russo said.

Holmes noted that some fans have found it creepy that Steve Rogers essentially made Peggy's future relationship with her husband, as well as their two children, disappear. But as the brothers both assured the audience, "They exist in a different timeline."

Read on for more revelations from the conversation, including the film's big deaths and the possibility of the Russos returning for more Marvel films.

Captain America may have been worthy of Thor’s hammer this whole time.

In one of the more memorable scenes from Joss Whedon's Avengers: Age of Ultron, the team hangs out as they attempt to lift Thor's (Chris Hemsworth) hammer Mjolnir. No one can do it, but for a moment it looks as though Steve Rogers might make it budge. To Thor's relief, Steve doesn't pick it up.

That moment pays off in Endgame, with Steve wielding the hammer fight Thanos (Josh Brolin).

When one young audience member who asked how Steve Rogers became worthy to wield Thor’s hammer, Joe Russo posed this question: “Do you think he was worthy in Ultron, but he just didn’t pick up the hammer so he didn’t hurt Thor’s feelings? Because clearly Thor’s very sensitive.”

Tony Stark’s final declaration, “I am Iron Man,” was the last scene they shot.

When Thanos believes he has won, he declares, "I am inevitable." Tony Stark (Robert Downey Jr.) responds with a line of his own, a call back to 2008's Iron Man: "I am Iron Man."

It was one of the biggest moments of the movie, but it wasn't always written that way.

“Initially, he didn’t say anything when he was facing off with Thanos — he just gets the stones and snaps. But we were in the edit room and it just felt a little flat,” Anthony Russo said.

The directors were brainstorming something Tony might say with editor Jeff Ford, “who is one of the heroes of the MCU — he’s edited more Marvel movies than anybody. He said, ‘What about 'I am Iron Man?’ And we were like, that’s it!”

The Russos saw the film's big deaths as letting Iron Man and Black Widow complete their journeys.

The job of Endgame, said Anthony Russo, was to find the most surprising but satisfying ending to the story arcs of all the characters who had been introduced over the course of 22 films. For both Tony Stark and Black Widow (Scarlett Johansson), their characters’ greatest moments were achieved by making the ultimate sacrifice.

“In a way, Tony was always fated to die,” Joe Russo said. “He was a futurist, always haunted by what was around the corner. [He went from] someone with an ego to someone who was completely selfless in the end. So that seemed like a very noble trajectory.”

And Black Widow also had an interesting trajectory, going from a trained assassin to superhero, and ultimately sacrificing herself so her friend Clint (Jeremy Renner) could live to be reunited with his family.

“Look at her relationship with Cap in Winter Soldier. She works in morally gray areas; he is black and white. They both assist each other in growing, and suddenly she starts to become a part of the community, a part of the family," said Joe Russo. "Ultimately, her sacrifice to save the family is a journey for her as a character — from an individual who is solely concerned about her own neck to someone who is willing to sacrifice for everyone.”

And if you’re concerned over the inequity between Stark’s funereal send-off and Black Widow’s more downscale one? Don’t be.

“She has a whole movie coming out," Joe Russo said, an assurance that received much applause from fans excited for the upcoming prequel.

It took a lot of work to get test audiences on board with Endgame's time travel.

Many of the debates that Endgame has sparked have been about the mechanics of time travel.

“Here’s the most important thing about time travel: It doesn’t exist,” Anthony Russo pointed out to an amused audience. “According by the rules of the movie, as stated by the Hulk and by The Ancient One, we go to great pains to tell you that Back to the Future is bullshit,” said Joe Russo much laughter.

But to the credit of Back to the Future, the Russos had trouble convincing audiences to buy into the multiverse theory because it required them to let go of the 1985 movie’s version of time travel.

Joe Russo continued: “Back to the Future has had such an incredible impact on pop culture over the past 40 years. If you’re trying to change the rules of time travel in a movie, unless you are super clear and point out that these are not the rules from Back to the Future, people just won’t understand it. We learned about this after three test screenings. We had to go back and add in more jokes about Back to the Future so that people would take our multiverse theory seriously.”

But they still have to spell out the multiverse theory.

Anthony Russo explained that, because they already had Ant-Man (Paul Rudd) exploring the Quantum Realm, they decided to delve into quantum physics for the Endgame version of time travel.

“The Hulk says if you’re in the present and you go back to the past, you cannot affect the present because it has already occurred. That now becomes your past. Right?,” said Joe Russo. “And if you’re [currently] in the past, this is now your present. And anything you do in that time shift would create a multiverse reality. It will create a new future, but it’s not going to affect your past.”

“But to be honest, talking about time travel is very similar to talking about god,” said Anthony Russo. “Everyone’s going to have a different understanding of what it is.” Joe Russo agreed: “At the end of the day, any thinking about time travel breaks down. What we tried to do was make sure the rules we were playing with [in regard to] time travel were honored by the plot of the movie.”

Stan Lee was everybody’s favorite.

MCU movies may have such gravitational pull that they can attract Oscar caliber actors to cameo in Endgame without the promise of any lines, but Marvel’s creator Stan Lee was the white-hot center of that universe.

“Stan was a ball of energy—very entertaining. He had the same joke every time he came to the set, which was, 'Why do I only get one line?' ” Joe Russo recalled of the legendary comic book creator, who died in November.

“The first time we’d ever worked with him was on Winter Soldier,” said Anthony Russo. “He showed up just cracking jokes, talking to everybody, shaking hands. To see everybody light up — when Stan was doing these cameos, there were always twice as many people on set. Everybody wanted to be there — we were in awe of him.”

There may be crossover movies in the Russo brothers’ future.

In addition to their new project Cherry with Tom Holland about an Army medic whose life unravels after returning from war, the Russos hinted at a potential series with the newly acquired X-Men franchise, which is now available to Marvel Studios for the first time thanks to the Disney-Fox merger.

“I think that the Universe is becoming more and more diverse, which we’re really proud of,” said Joe, “especially now that Fox characters are coming into the Universe. There are a lot of stories to tell.”

For more from the Russo Bros., read our Q&A with them, in which they discuss seeking Downey's blessing for the death of Tony Stark.