Max Casella Recalls 'Broke' Days Before Playing Vinnie on Doogie Howser : 'Changed My Life Overnight'

Jamie McCarthy/Getty Images Max Casella

It's been more than 30 years since Max Casella arrived on the sitcom Doogie Howser, M.D. to play the loyal best friend to Neil Patrick Harris' child prodigy. But the actor still remembers the thrill of booking the role, his first big job.

"It changed my life overnight," Casella, 55, tells PEOPLE in this week's issue.

At the time, Casella was 21, living with his mom and trying to find work in New York City. "I was working in an off-Broadway theater as a janitor. We were broke. I had to jump the turnstile to get on the subway to get to work," he says.

Suddenly, the theater kid with aspirations of Shakespeare found himself living in Hollywood and able to buy his first apartment. "I hadn't been anywhere, never had a girlfriend, never lived on my own, never did any of those things," he notes.

Playing Vinnie jettisoned the star back in time to high school, but Casella was used to playing juvenile characters, particularly "because I looked so young," he says. He and Harris had "great chemistry" on Doogie, he says, but off-camera their age difference proved more noticeable, particularly as Harris was just 16 when the popular sitcom debuted in 1989.

"Neil was super professional and carrying the whole show at such a young age," Casella recalls. "It's funny, because he was playing this very responsible grown-up character, and I was playing this total immature goofball bouncing off the walls. And then when the cameras were off, he would actually be bouncing off the walls doing magic tricks, juggling or being a kid. And I'd be off brooding or sulking over a cigarette in the corner, because I'm six years older than him. We were total opposites."

ABC Photo Archives/Disney General Entertainment Content via Getty Images Max Casella as Vinnie on 'Doogie Howser'

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The years following Doogie proved tumultuous for Casella. In his late 20s, the actor found himself struggling to book the kind of roles he had aspired to. "Show business can just step all over your heart," he says matter-of-factly.

Casella experienced a specific rabbit hole of fame: "You go out to LA, get on TV show, get famous and then you're known for this show for a while. I was in my 20s, I was playing kids, prepubescent characters. And then I grew out of that and grew up into manhood, and I had to start all over again, because it wasn't a smooth transition like it is for a lucky few people."

Casella, who found early momentum in films like 1992's Newsies with Christian Bale and 1996's Sgt. Bilko with Steve Martin, felt at the time it was movie stardom or bust. But in the process, "I got away from all the studying I had done about acting," he admits.

In his early 30s, the discouraged actor returned to New York, starred in The Lion King on Broadway and took up acting lessons again, essentially starting from scratch. "I took a beginner's class with people who had never worked," he says, "and I studied for seven years."

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Casella then began booking more meaningful work. "I started to bring what I was learning in class to The Sopranos" playing gangster Benny Fazio, he says. "And suddenly acting became fun and my confidence went up and I fell in love with the craft of it, which is where I'm at to this day."

The dad of two has since appeared in movies like Jackie, Inside Llewyn Davis and Late Night as well as TV shows including Boardwalk Empire, Ray Donovan, Vinyl and The Marvelous Mrs. Maisel.

Brian Douglas/Paramount+ Max Casella on 'Tulsa King'

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Now he stars opposite Sylvester Stallone in the new mob drama Tulsa King (premiering Sunday on Paramount+), playing a recovering gangster hiding out in Oklahoma. Casella calls the project "the greatest experience of my career" — in part due to the richly developed character he plays.

In Tulsa King, Casella portrays "a guy who's been running his whole life and pretending to be something he's not," who is forced to face his past when Stallone's mobster suddenly moves to town.

Working with Stallone "was like a dream come true," Casella says. "I had his poster on my wall when I was 10. He was everything you would want him to be."

For more on Max Casella, pick up the latest issue of PEOPLE, on newsstands Friday, or subscribe here. 

Tulsa King premieres Sunday at 9 p.m. ET on Paramount+.