The Australian actor takes on the role of Supergirl, aka Kara Zor-El, in one of her biggest projects to date, and Nogueira told Rolling Stone there was no one else who could have done it as well as Alcock.
"That was the whole thing - don't make a focus-grouped movie. Make something that feels honest, and that I always think appeals to more people," she said.
"And that's just who Milly is. She's perfectly cast."
Supergirl: Woman of Tomorrow director Craig Gillespie added: "She's so beautifully candid and honest and open. She's that as a person, and you see it in her performances. She channels that in the best way."
While there are already positive reviews from critics, there are also those who won't make a secret of their dislike of Supergirl.
Nogueira said she tries to "ignore any culture-war aspect".
"I do believe all of these people love these characters a lot, and they mean a lot to them, and have for years, decades maybe. I just wanted to be like, I think this one's for everyone.
"We're just gonna tell a good story, and it's not gonna be an us-versus-them kind of a situation. I want everyone to like it."
At one point in the film, Supergirl is seen going through daily actions, such as going to the bathroom and throwing up.
Asked why he included that, Gillespie mused: "There's just a reality to it that I like, there's a casualness, and it just grounds them more in the human world. It's like, they have to have these functions. They have bodies."
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