Almost a decade after their Marvel Cinematic Universe debut on Captain America: The First Avenger, Christopher Markus and Stephen McFeely would go on to pen Avengers: Endgame, the culmination of 11 years of storytelling. At Comic-Con Intentional in San Diego, the writers stopped by Hall H to discuss the process of writing the blockbuster film.

"Endgame was 99% done before Infinity War came out," Markus shared.

"We learned that people really liked Spider-Man," McFeely added.

When the moderator brought up a scene cut from the film, Markus offered a bit more detail about it. "We didn't lose it. We took it out!" he explained. "It was partly inspired by Kevin Fiege... he came in to the conference room in Atlanta, where we were trapped for years, and he had brought a bunch of comics panels with him... we were pretty far down the road... and he wanted to make sure we didn't lose the psychedelic nature of the Starlin universe... and so we stuck the Living Tribunal in the movie... So, as Thanos and Dr. Strange came to blows, Doctor Strange blew Thanos' mind and send him through the mind-scape."

"Not unlike the Ancient One did to Doctor Strange in Doctor Strange," McFeely added. He explained this scene would have shown Thanos all his crimes against the universe. "In the end, he would have been dumped in front of the Living Tribunal, who judged him guilty."

"You get handed these two movies and your first reaction is panic, because that's a lot of work, and if it's bad, you'll be very embarrassed one you and then you have to wait around because you're going to be embarrassed next year," Markus recalled. "[Feige] said just stick the landing... If that means cutting the Guardians out of this movie, if that means shrinking it to make it more manageable... do it."

McFeely revealed Thor's stroyline was almost much different. "He had to go get the ax but he was fighting a serpent," he admitted.

"It was insufficiently character-based," Markus added.

"Marvel knows how to foster the writing of a movie... They're not a panic-based studio," Markus shared. He explained that some studios freak out at the first draft and suggested they would have been fired by another studio. However, Marvel worked with them on their drafts to build up a better movie.

"We wanted to take your expectations early, but that comes from Thanos as an omnipotent... villain," McFeely said of Thanos' death in Endgame. "We spent a couple months going, 'What do we do as this guy?'" Fortunately, in a moment of frustration, someone asked them: "'Why can't we just freaking kill him?'" And so they did.

"It's delightful to be given that kind of leeway, for one thing. You say five years later, in your head, you go, 'Oh, we're telling a real story,'" Markus shared. "It is a chance to really see what -- this whole movie, all of Endgame, is about the effects of the last movie... put failure dead-center int he lives of the superheroes and to crush them a little bit... for five years, they have lived with abject failure."

As to Professor Hulk, Markus revealed he almost appeared earlier, in Infinity War. "These movies are worked on up until release," he explained. "We wrote it, we shot it and then went on to shoot most of Endgame. He achieved with the union Hulk while inside the Hulk... burst out and kicked Cull Obsidian's ass. And it didn't work! It was completely the wrong tone for that moment in the movie. It was this moment of victory while we were headed toward defeat... but we had already shot Endgame, where he was already Smart Hulk..."

"You don't have to explain as much as you think you have to," he added. "Okay, he's culmination of Hulk and Banner, just give me a crumb to hold on to... Part of the challenge and part of the pleasure... of these two movies... is how do you tell everything you need to tell... and keep it light enough you're not bogging it down?"

"A really funny scene where Hulk is eating pancakes... and it's a scene I haven't seen in a superhero movie," he said.

"We knew we wanted to go back. What other franchise would allow us to do what we wanted to do?" McFeely asked, pointing out how the film goes back to the MCU's past installments.

Asked how Ant-Man survived the Snap and if it had to do with being in the Quantum Realm, Markus had an interesting response. "I think he was coin-flip lucky, because the Stones do not differentiate between realms," he said.

"We had to boil our time rules down... and it came down to the Stones. The Stones are going to hold the universe in place," Markus explained. "We needed to someone to explain it, and who's the most entertaining explainer in the Marvel Universe? Let's get Tilda [Swinton]!"

"We showed it to some test audiences and every time Nebula shot old Nebula," they didn't get it, McFeely revealed. "We had to go out of our way to exposit the hell out of it."

"It clearly exists, because Doctor Strange has talked all about it," MeFeely said. "I have no idea where Kevin Feige is going... but I'm a big believer in stakes." This is why he thinks the MCU will not introduce an alternate-world Tony Stark or Natasha Romanoff as a replacement for their dead counterparts.

"Once we figured out the track we wanted Thor to be on, that he was really going to crumble... that the only person who could put him together -- we wrote a scene with him and Jane," Markus recalled. "Jane is not the person to put him back together. Odin is not the person to put him back together... It had to be his mother."

"We've been with Howard a long time... and to be able to bring him into a kind of closure with Tony was satisfying," Markus shared. "It was very satisfying to take these two and put them together and have them to have a real honest, human moment... To be able to find that emotional core in... the most bonkers section of the movie, was really good."

McFeely touched on Hawkeye and Black Widow's scene before her death. "That's the second or third iteration of that. A lot of that choreography is Joe and Anthony [Russo] -- they're both fighting to be the hero," McFeely said. "They both feel they're the ones to do it for a number of reasons."

On the topic of Red Skull, Markus revealed Marvel Studios composed a "manifesto" to explore options for Endgame from he earlier films. Red Skull was one of them. "We rote a big document that was everything you could possibly do in these movies," he revealed. "In there, is the Red Skull is clearly alive... He's awesome! Let's find a way to use him. Let's tie this whole thing together... We did send him to space and send him a way very much on purpose..." He added that they didn't know where he was going when Red Skull disappeared in The First Avenger.

"There's a fascinating one-act play to be written of Captain America going to Red Skull," Markus joked. "Never wrote it. In the manifesto document, there was one theory that Cap who goes to space... and he has to collaborate with Red Skull to get the Stone."

"In one, he -- being Thanos -- knowing it would take Nebula some time to rejigger the time machine... so he went to his present-day Earth and wiped it out and killed the Avengers and then Nebula would turn on the time machine, he would walk through the time machine with" the body of an Avenger he had killed "and say, 'I killed you again. What are you going to do?'" Markus revealed.

They also initially had the Snapped Avengers return to the Avengers compound. "We had one where they all come back to the compound, and then we cut five minutes and they're all eating pizza... and it was terrible!" McFeely said with a laugh.

"Everybody came back to the point they disappeared, so then they had to get to our battle field, so that gave s a window of time where things still had to happen," Markus said. "We couldn't bring them in until we had Thanos in."

"When he doesn't talk, you are crushed and [Robert Downey Jr.] knew that... and he said I want to do much, much less, and he was right," McFeely said of Tony's death scene.

The moderator brought up the theory that two Captain Americas exist simultaneously in the same timeline: one from the original Captain America trilogy and another "playing golf" on the beach with Peggy. "Cap doesn't golf," Markus argued. "There are varying theories. I am much very entertained by that theory."

Directed by Joe and Anthony Russo, Avengers: Endgame stars Robert Downey Jr. as Iron Man, Chris Evans as Captain America, Mark Ruffalo as Bruce Banner, Chris Hemsworth as Thor, Scarlett Johansson as Black Widow, Jeremy Renner as Hawkeye, Brie Larson as Captain Marvel, Paul Rudd as Ant-Man, Don Cheadle as War Machine, Karen Gillan as Nebula, Danai Gurira as Okoye and Bradley Cooper as Rocket, with Gwyneth Paltrow Pepper Potts, Jon Favreau as Happy Hogan, Benedict Wong as Wong, Tessa Thompson as Valkyrie and Josh Brolin as Thanos. The film is set for release on Digital HD on July 30 and Blu-ray and 4K UHD on August 13.