“I still have them on a piece of paper somewhere,” Finn says, before naming a few: charisma, quick wit, immense intelligence.

They chose Robert Downey Jr. with little doubt in their minds that he was right for the part, though the announcement was met with raised eyebrows from the public. Downey’s relatively recent struggles with substance abuse had affected his mainstream career, such that an Entertainment Weekly writer felt the need to reassure readers of the casting decision: “No, really, it makes sense.”

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The risk paid off. Downey’s performance as the morally torn superhero anchors the Marvel Cinematic Universe’s Infinity Saga, which began with 2008′s “Iron Man” and concluded 21 films later with last month’s box-office behemoth, “Avengers: Endgame.” It’s difficult to imagine anyone but him in that role — a statement that could extend to any of the heroes, really.

That’s largely thanks to Finn, who took on the gargantuan task of casting every actor who appears in the MCU (aside from those in “The Incredible Hulk,” released a month after “Iron Man”). That amounts to more than a thousand roles overall, she says, ranging from characters as high-profile as Captain America to those as minor as his background dancers. The job — which Finn held for the first five MCU films alongside Randi Hiller, who now heads casting for live-action projects at Walt Disney Studios — calls for a certain prescience, the ability to predict what sort of traits an actor would one day be asked to exhibit in films that have yet to be written.

Finn hadn’t been aware of that responsibility early on: “It wasn’t until we got into ‘Captain America’ and ‘Thor’ that Kevin Feige even mentioned the Avengers films,” she says of the Marvel Studios president, who consults on casting decisions along with fellow executives and the directors of each film. Finn quickly learned that the interconnected nature of the MCU meant she’d have to find actors who were ready to pop up in any scene, and who could vibe with almost anyone else on screen.

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When they cast Paul Bettany as the voice of Tony Stark’s operating system J.A.R.V.I.S., for example, Finn says she had no clue J.A.R.V.I.S. would later be injected into a synthetic body named Vision, a physical role Bettany also took on. Later, while casting Letitia Wright as the Wakandan princess Shuri in “Black Panther,” they had a feeling Shuri might share a scene with Tony down the road and had to anticipate the on-screen chemistry Wright would have with Downey. (Wright actually interacted with Mark Ruffalo in that “Avengers: Infinity War” scene, further proving how unpredictable the MCU can be.)

And then there’s the Mad Titan, of course.

“When Thanos first appeared in the Marvel movies, it was really just a couple lines of dialogue. The character was shadowy and not that visible,” Finn says. “To see him go from a few lines to being one of the most iconic villains of all time in movie history, I don’t think Josh [Brolin] even knew how his character was going to be represented on screen, or the impact he would possibly have.”

Scheduling is its own beast. Coordinated by a producer on “Infinity War,” aligning the A-listers’ schedules for the final two Avengers movies was "a tremendous feat that probably hasn’t been done ever,” she says. But she points out that production schedules can influence the visual aspect of casting, too. When it came time to figure out who would be the face of the rebooted Spider-Man franchise, Finn says, they had to keep in mind that Peter Parker would be introduced in 2016′s “Captain America: Civil War,” which was shot a year before “Spider-Man: Homecoming” but whose events took place immediately before.

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“We had to find a young actor who hopefully wasn’t going to grow six inches in the intervening time period,” Finn laughs. “Poor Tom Holland was so antsy and ready to start shooting.”

Casting directors are often brought on between 12 to 16 weeks before a film starts shooting, Finn says, but with all the extra considerations that accompany an MCU project, she has sometimes started the search up to a year or two in advance. At the end of the day, it’s all about finding actors who will do the widely beloved comic book characters justice. That takes time, especially with projects of this scale.

The search for a Thor was especially “daunting,” according to Finn, given that they had to find an actor who could embody “both the qualities of an Asgardian god and be a relatable figure on Earth.” At the time, Chris Hemsworth was most notable for playing Captain Kirk’s father in the opening scene of J.J. Abrams’s “Star Trek,” and Tom Hiddleston, who would play Thor’s brother Loki, had mostly worked in England. Finn recalls a 2009 Vulture article titled “Marvel Rolls Dice, Casts No-names for ‘Thor.’ ”

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“I’ve lost a lot of sleep over the years,” Finn says. The no-name Hemsworth worked out quite well — since “Thor: Ragnarok,” critics have praised the actor’s decision to lean into his comedic impulses.

As for the remaining Avengers? Chris Evans’s reluctance to play Captain America proved why he was the perfect person for the role: “He has a humility, a moral compass that is tangible and that the audience can really feel with him,” Finn says.

Scarlett Johansson, who plays the former assassin Black Widow, wasn’t known as an action hero but “embraced that challenge . . . with strength and an endearing quality that really resonated with us.”

Finn has known Ruffalo since their off-Broadway days and was therefore “keyed into his sensibility as an actor, his intelligence, his subtlety” — traits they wanted to bring out in Bruce Banner.

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And Jeremy Renner, who plays Hawkeye, “was someone who was always in our mind.”

The process is a little less intensive when it comes to voice actors, as Finn’s team can extract tape from other films in lieu of in-person auditions. Vin Diesel’s performance in “The Iron Giant” spoke to the qualities she wanted to highlight in the treelike being from the Guardians of the Galaxy franchise who famously only pronounces, “I am Groot.” Bradley Cooper famously altered the way he spoke as Jackson Maine in “A Star Is Born,” but the “quality and timber of his voice” in previous roles let Finn know he could bring “a lot of humor and pathos” to Rocket, the genetically enhanced raccoon.

Finn continues to gush about the MCU’s talent, rattling off the names of several others, from Brie Larson, who had been in talks to play Captain Marvel since showing off her action hero side in 2017′s “Kong: Skull Island,” to, really, every single actor in last year’s “Black Panther.”

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“I could go on and on and say how proud I am,” Finn says.

These recent additions will carry future MCU films, given that “Endgame” serves to finish off the arcs of several original Avengers. Finn is currently tied to three future projects, “Guardians of the Galaxy Vol. 3,” “Black Widow” and “The Eternals,” none of which she will comment on beyond stating that “there is a real desire and movement toward greater diversity across all of the films, in front of and behind the scenes.” (Angelina Jolie and Kumail Nanjiani have reportedly been cast in major roles in “The Eternals,” which will be directed by Chloé Zhao, whose second feature, “The Rider,” appeared on a number of film critics’ top 10 lists last year.)

Finn laughs when asked whether she’ll continue to work on all the Marvel projects recently announced in Disney and Fox’s theatrical release schedule, which runs through 2027.

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“Hopefully they’ll have me,” she says. “I try to have an idea of what’s coming up, but I don’t think I know what’s coming up until 2027. I’d never sleep if I did.”

clarification This article now reflects that Sarah Finn worked alongside casting director Randi Hiller on the Marvel Cinematic Universe films from "Iron Man" to "The Avengers."