To prepare, the writers behind Captain America: Civil War crafted a 60-page document featuring every scenario they could conceive of for these two films, and asked Marvel Studios head Kevin Feige and his braintrust to circle things they liked. The document contained things both "big and small." One of the more fascinating items on that list is a title comic book fans will recognize: House of M.

The 2005 storyline saw the Scarlet Witch remake reality into one in which mutants such as the X-Men villain Magneto controlled the world. in the comic, the Avengers and X-Men must work to set reality back to the way it's meant to be. House of M was just one of hundreds of ideas that came during the brainstorming for what would become Avengers: Infinity War and Avengers: Endgame. It all happened years before Disney's acquisition of 21st Century Fox gave Marvel Studios access to the X-Men.

In other words, had elements of House of M been used, it's clear the X-Men wouldn't have been involved, but the anecdote shows how deep the screenwriters went to craft their two Avengers movies, which stand as the No. 2 (Endgame) and No. 5 (Infinity War) films of all time globally.

While the duo have left Marvel behind (for now), McFeely does single out House of M as something that could interest him in the future.

"I think House of M would be awesome. But you've got to earn it," he tells The Hollywood Reporter.

In a conversation with THR, the duo also discuss the logistics of Tony Stark's funeral scene, the moment they realized Thanos would die in the first act, and how they kept track of their massive cast's deals.

What was on that 60-page document you crafted before writing Infinity War and Endgame? Things like "Captain America catches Thor's hammer"? Things bigger than that? Smaller?

Stephen McFeely: Big and small. House of M.

Christopher Markus: House of M. I remember going through several scenarios of Nova. Thanos could come and kill all the Nova Corps, except one guy. Richard Rider, who then becomes … I think in that scenario he became the herald, kind of like the Hulk was [in Infinity War when he crashed into the Sanctum Sanctorum]. Someone who could come out and tell people. We had everything. Literally every variation we could think of or that was sitting there in the comics untapped. With absolutely no value placed next to them.

McFeely: We may have inherently said this is a five-star idea, this is a one-star idea, but we tried not to present that to the group.

We all want to know where you keep that document now.

Markus: It exists somewhere.

McFeely: It's on our laptops. Kevin has it, I'm sure.

There are visual Easter eggs in Endgame, like the Hulk holding up a structure. Is that in the document? Do you have certain ones you want to hit?

Markus: They come from all around. I remember at one point, we were all in Atlanta already in pre-production and Kevin came down for one of his periodic site visits. He had a bunch of comic panels with him on his computer. That may well have been one of them, that cover. I think he had a few of Thanos.

McFeely: Mephisto. "Could we make sure Ebony Maw is more Mephisto-y?" Because that's a whole kettle of fish. Mephisto is the devil.

Markus: He's one of several Marvel devils. Because Marvel also has a Satan.

The Russos have talked about how star salaries weighed on their budget. They blocked the movie to shorten the time stars spent on set. Do you think about that in the writing? Do you think about how many pages Tony Stark is on, because Robert Downey Jr. is expensive?

Markus: Not the financial aspect of it. On the now increasingly legendary set of baseball cards that had all of the Marvel people on it, you could flip it over and see how much were they contracted for or would they require a new contract.

McFeely: We didn't have salaries [on the back].

Markus: It didn't necessarily dictate anything. If they didn't have a contract and we needed them, they'd just make a new deal. But it was kind of interesting to see.

McFeely: It was helpful when it said "run of show." Oh, this person is committed to this movie and we won't have to play any scheduling games.

When you write the Tony Stark funeral scene, do you specify every character you want individually there, or do you just write "all characters we can get"?

Markus: We specified everybody [individually]. I'm wondering if we specified anyone who didn't end up being in the film.

McFeely: No, I don't think so. If you got to the third act of the movie and you survived you end up at the funeral, and some people who didn't even do that. Nick Fury, Maria Hill, Harley Keener — the kid from Iron Man 3.

People are loving that.

McFeely: We certainly debated it. "I recognize all these people, but this kid has grown like a weed over the past five years."

Markus: It's weird, because it is a sequence that's all about recognition. Oh my god, look at all those people I know, and you hit this kid and you're like OK…

Kevin Feige is the one who suggested using time travel in Endgame. Was that an "ah-ha!" moment where suddenly you knew how to beat Thanos? Were there other ways you had written, or were you hitting your heads trying to figure it out how the Avengers win this?

Markus: We hit our heads for quite a while.

McFeely: Remember, he is omnipotent. He is omniscient. It is ridiculous how much power he has at the opening of the movie, so for a good solid three weeks, we are trying to figure out, what is movie two with a character with that much power? At one point, I think Trinh Tran, our executive producer, in frustration said, "God, I really wish we could just kill him." We all went, "slow down. What does that mean? That's interesting." It's absolutely within his character. When we asked, "Why would he let you?" Because he did what he wanted to do. It's strong for him to do that. We've been at it a long time and that's the kind of thing I beat myself up for not thinking of earlier. If I'm being consistent to his character, this is on the table.

Markus: Time travel pops up in your head early in almost any difficult situation. Well, if I travel back in time, this date would have gone much better. Generally in a movie, "eh, that's way to easy." We couldn't have Tony just invents a time machine. But delving into Ant-Man, who we hadn't used because he had a movie coming and we didn't want to taint Ant-Man and the Wasp any more than we did at the very end. ... We were planning on Ant-Man because we had access to him in the second movie, and the fact that he was bringing a whole subset of technology that did have something to do with a different concept of time was like a birthday present.

McFeely: We brought in physicists to ask, "Give me quantum mechanics for dummies."

Markus: And then we went, "dumber."

McFeely: Particles at that level, the same particle can exist at two places at the same time. That could be legitimately how it works.

Markus: Solid enough for comic books.

How much of the world-building did you do for an Earth in which half the population is gone?

McFeely: There are very few parts for non-movie stars where you get the wider effect of the snap. We knew we wanted to give you a little taste, but boy it wasn't going to be much, because Scott is going to come back with the answer.

Markus: We have the weird burden of having the deepest bench of movie stars in history, where your third banana supporting actor is the star of their own franchise. There's no room for a newspaper guy.

McFeely: His [Christopher Markus] cameo is half a second long, my wife is buried in the background. We can't actually find room for people. Long way of saying, it was going to be in that first section just before Scott comes back and we didn't go as deep as The Leftovers I assume did.

Thanks to the new Spider-Man: Far From Home trailer, we know there are now multiverses for Marvel to play with.

McFeely: Do we?

True, it's Mysterio. So he could be lying.

McFeely: That's the thing, consider your source.

But when you are leaving Marvel with these types of toys, how do you know, "OK, it really is OK for us to open the door for a multiverse" or other things like that? Does Feige have a glint in his eye when you tell him?

Markus: He gets a glint. He gets excited, especially when a big swing is proposed. Something that will kind of staff stuff, he does seem to respond to that. "Take down SHIELD. Civil War." He sees the value in breaking the toys. He was always pressing for a good sized time jump and to make it permanent. "Do it. We'll deal with it and it will just make it more interesting. Why would you undo it and go back to zero?" If we went back five years and undid it, that's five-and-a-half hours of movie that sort of half no point. You loop back around to the beginning and it never happened.

McFeely: It's really important to own it.

You've given Tom Holland credit for his Infinity War death scene and coming up with its most heartbreaking aspects. Did you have time to watch his death scene and shadow that in Tony's final moments in Endgame?

McFeely: I honestly don't remember.

Markus: It was shot before Infinity War came out. I don't know if we'd seen dailies or not. We probably had.

McFeely: We'd seen it on the day, because everyone got choked up, even on set.

Markus: I think that had a bearing on bringing Peter back around to be one of the few people who gets that face time with him. But it wasn't a direct callback.

The Russos always say the Secret Wars storyline is what would bring them back to Marvel. Do you have a project like that, which could pull you back?

Markus: There are all sorts of characters that would be fun. Joe and Anthony keep saying Secret Wars and I keep going, "really, you want to take on another giant multi-character [crossover]?" I would personally be more drawn to setting up a single character again and taking them from the beginning.

McFeely: I think House of M would be awesome. But you've got to earn it.

Ten years from now.

McFeely: It might take awhile. And it seems hard.

Markus: It doesn't really seem within her [Scarlet Witch's] skillset.

McFeely: We took her skillset away. She had mind control stuff early.

McFeely: Somewhere there's a great Moon Knight movie, but it's complicated.

You are continuing to work with the Russo Bros. on non-Marvel projects. What sort of things are grabbing your attention these days?

McFeely: We still can't talk about it. There's a true-life story we turned in earlier this year we're really excited about. It's very political and we're close to casting that up. We are starting a script called Electric State, it's for Andy Muschietti. That's a weird, post-apocalyptic girl and a robot movie. Part of us wants to create another world. What's the next Marvel world for somebody else? But that's a heavy lift, and I'm tired.

It's not easy to be Stan Lee.

Markus: It would be healthy to alternate big and small. This political thing is quite small, Electric State is quite big. There's a vast world of … do you even call it television anymore if its streaming? Do you call it TV? Having been given, I don't know how many collective hours by Marvel now to tell an ongoing story, that real estate is very attractive and much more common in TV. That's a path that should be explored.

McFeely: As opposed to network television where you rarely have solid footing to plan for your last episode, it seems like in streaming you can say, "It's going to be eight episodes. That's it." If you know that, you can craft a nice landing. You can really stick it. That's interesting to us. Long-form storytelling with an end.

Markus: Now that I think about it, we've been given about 12 hours by Marvel, which is about the run of a streaming series. Granted, we had 11 years to do it and more money than is justifiable.

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For more from Markus and McFeely, see what happened when they tackled the internet's most persistent Avengers: Endgame fan theories.