Sarah Finn is no stranger to the world of casting. Many will know her as the casting director for Marvel — she’s assembled every film in the MCU since 2008’s “Iron Man.”

Her credits also include “Everything, Everywhere All at Once,” 2019’s “The Lion King” and “Chef.” Searchlight’s “Wild Horse Nine,” now in theaters, reunites her with “Seven Psychopaths” director and writer Martin McDonagh.

The film stars John Malkovich as Chris and Sam Rockwell as Lee, who play CIA agents dispatched from Santiago to Easter Island by their bureau chief, MJ (Steve Buscemi). Among the Island’s iconic statues, and as the longtime partners wrestle with their dark pasts and present conspiracies, Chris’s newfound bond with a pair of rebellious students threatens to send everyone’s trust to this remote island paradise sideways.

Finn spoke with Variety to discuss her process for casting the “Wild Horse Nine,” why Chadwick Boseman’s audition still holds a special place in her heart and the satisfaction of casting Marvel films.

What’s the process of casting a film like this where you have roles offered, and you have to build out the rest of the cast?

Sam and Martin go back to “Seven Psychopaths,” and before that, Sam had done one of Martin’s plays. So it’s a mix of relationships and then curating and building the cast together, understanding the world, understanding Martin’s vision.

We were able to dive into Chilean history and the Chilean talent pool a few months before we started active casting. We’re going on two tracks: looking there and discovering new talent, and talking about every role, from the Steve Buscemi part and how every single one of them was its own journey to put the pieces together in a thoughtful and careful way for maximum impact.

John is phenomenal in this. What was it about him that made him right for this?

His entire body of work speaks to his talent and his history as an actor. The founding of the Steppenwolf Theater Company speaks to his ethos as an actor, and his talent needs no explanation. However, I do think that this is a part that he was uniquely suited to play at this moment in his life and sometimes that just arrives.

He’s always had an effortless style of acting, but the way that he’s able to move from one aspect of this character’s personality to another in a moment and show a full range of humanity, feels very special. As much as we’ve seen from Malkovich. I don’t think we’ve seen this kind of performance from him before.

I loved how he delivers that wit. What was that like seeing him bring Martin’s words to life in that way?

It’s especially gratifying when a film and a performance can exceed our expectations. There were qualities that John brought in moments that I could not have anticipated. But yes, knowing he had the humor, also the danger and the darkness. There’s so much regret in the story. There’s a sense of atonement, but as an audience, we need to care for him. Martin met with him, and they had conversations about the character and the role, so they could understand together how John might bring this character to life. From a casting standpoint, we had to explore every angle of the character and every angle of the actor and make sure that all of these colors were going to come to life on the screen, as well as the chemistry that we were hoping to achieve between the characters and among the whole cast.

Enter Sam Rockwell, who worked with Martin before. If that chemistry didn’t work, the film wouldn’t have worked. The Arkansas moment wouldn’t have worked.

It’s so important, and there’s some moral ambiguity. Everyone in this cast is funny. The writing is funny. Martin is funny. So even if that’s deeply, darkly funny, they all have to understand it. Sam is a master at that. He has many different qualities that he can bring to the fore in this part: his relationship with his wife, his boss, his friend and then exploring what’s true and what’s not for his character.

I’ve known Sam a very long time. We’re used to seeing him in these very kinetic, vibrant performances and in this role, he’s so subtle. He carries so much emotion in the silences. I think he helps us understand so much about what Malkovich’s character is going through, in the way that he holds those moments and I thought it was so thrilling to see him in this way because he was able to do so much.

You mentioned going into the Chilean casting pool. How did you find Mariana di Girolamo?

I got to meet Mariana in 2019 after Pablo Larraín’s film “Ema.” I was so taken with her. And I’d been looking for something for a while, so the script arrived and here’s this amazing Chilean role.

We saw hundreds of people. We auditioned a lot of phenomenal actresses. But she was top of mind because I had met her, and I had a feeling not only for her range and her talent, but for her warmth. I knew that also was going to be something we wanted to feel from her character in this film. There’s a sense of idealism, purity, hopefulness and she goes on her own full journey. It was challenging to find a Chilean actress who could handle the language, which is really specific, unique, demanding and has to be mastered. I was thrilled that she won the part and was able to show everyone how talented she is.

What’s the key to working with Martin?

You just fall in love with the writing and his vision and the world. That’s what energizes me. There was not one part that didn’t feel special. There was not one character that didn’t have its place. And every line that he writes is so intentional that it is really very special to get to work with him on these films.

How did Parker Posey get that part of Sam’s wife in the film?

That was a fun role to cast, and we could think very widely about who could play that part, but I did think it was important to have absolutely the right person. Parker is her own incredibly talented, idiosyncratic, unique performer in the way that everyone else in the cast is. But she also shares the same work ethic. For her to fly to Easter Island to read lines off camera for the phone call with Sam Rockwell is not something that happens that often in this world anymore.

I have to ask you about the Marvel process, which is shrouded in so much secrecy. We had the D23 cast reveal with Sadie Sink; what is it like to see that happen on such a big scale?

I’m used to it at this point, and I understand that it does protect the process. It protects the actors, the filmmakers and everybody throughout the creative process and it really is for the fans. As much as it’s frustrating, I think that it’s meant to increase the surprise elements, the enjoyment and fun of it, because it’s nice to be surprised. I’m doing my best, working as closely as I can with the filmmakers to achieve their vision and wanting to get it right for the fans. But I know that there’s always going to be a lot of points of view, and I think D23 was an exciting rollout this year and we’ll keep going. That’s the biggest cast we’ve put together. With this much expectation, I definitely slept better because fan expectations were really high and I felt I feel the pressure. I can’t not.

You have launched so many careers. Is there an audition tape or an audition that has really stuck with you?

The one that comes to mind, this is very emotionally loaded for me, but it was Chadwick Boseman, who I knew. I met him in 2005. He came in to audition for Drax, the Destroyer in “Guardians of the Galaxy” and it was right around this time he was booking leads and movies. He missed the humor in Drax, but he played him like a king. It was really from that moment that I think I started tracing that for Black Panther.

How does it feel now that people are starting to begin to understand your process?

The biggest misconception is that people think that casting happens very easily. It’s hours of painstaking work: getting out there, covering ground and reading as many people as we can. There’s a work element that people don’t factor in because when it does land perfectly, and you cast the right person, it seems so obvious. I think it’s easy to misunderstand how much thought, experimentation and exploration and hard work went into arriving at that perfect choice.

This interview has been condensed and edited for length and clarity.