There is a real Jack Castello actor who appeared in some Spanish films in the '20s and '30s. There are also two Jack Costellos (spelled with an "o" instead of an "a"), one who narrated films in the '40s and '50s and one who is younger and recently appeared in 2015's Eddie the Eagle. But those are similarities in name only, not in character. The Jack of Hollywood is fictional and more of a stand-in for many young, naive, wanna-be actors fresh from war and wanting to do something different with their lives.
Jack is played by David Corenswet, whom Murphy previously cast in his show The Politician (Corenswet played River). Corenswet told Refinery29's Ariana Romero that he had heard Murphy was working on Hollywood after realising his character on The Politician wasn't coming back in season 2. Still, Murphy was interested in him for another role, and while Corenswet hoped that role would be in Hollywood, he didn't think it was likely.
“I was at home in Philadelphia with my dad and my sister and I showed it to them,” Corenswet says of the initial announcement. “We all kinda went, This would be too good to be true. It couldn’t possibly be what he wants me for.”
Certainly Corenswet's old-Hollywood looks made for the perfect transition to playing someone from that era, but Corenswet told Elle magazine that he wanted to make sure he was right for the role of Jack for more reasons than just his appearance. "[Ryan Murphy and I] sat down and had a long conversation, which basically started with me asking, 'Why me? Why do you want me to do it?'" Corenswet told Elle. "Somehow, he knew that this time period and this story about a young, optimistic kid who really cares about doing something that matters was right up my alley."
And though he didn't have a real life person to base Jack on specifically, Corenswet did do his research. THR reported that he watched 1940s films (and in many cases re-watched, because he'd been a fan of the era growing up). Entertainment Weekly specified that he dug into Jimmy Stewart and Gene Kelly films specifically, because his own character "has a certain optimism [and] a lack of jadedness" like many Stewart and Kelly characters do.
That optimism and lack of jadedness also translates into Jack not being quick to judge others around him. "I think one of the interesting things about him is that he is blind, for the most part, to the injustices — they haven't affected him personally, so he isn't aware of them, but also he hasn't internalised the prejudices of the society around him," Corenswet told THR. "So, when he meets Archie [a black, gay, aspiring screenwriter], all he sees is another young guy who's struggling to come up and who's got to make compromises."
Not many people in or out of the industry in the '40s were as blindly accepting as Jack (at least publicly so), so it was necessary to make him fictional to explore a storyline that was rewriting the often racist, sexist, homophobic beginnings of the Golden Era of Hollywood, and Corenswet rose to the challenge.