Katselas, who founded the Beverly Hills Playhouse acting school in 1978, died of heart failure Friday at Cedars-Sinai Medical Center in Los Angeles, said Allen Barton, executive director of the playhouse.
For decades, Emmy-winning actress Doris Roberts studied with Katselas and was a regular at his weekly master class.
"I am the actress I am because of him. I am the human being I am because of him," Roberts told The Times. "He was an original, extraordinary.
"I learned something new every Saturday, even when I was working on 'Everybody Loves Raymond,' " she said. "He had such insights into people. He was so capable of finding the kernel in you that was stopping you from succeeding."
In an interview last year on “Inside the Actors Studio,” Pfeiffer said he taught actors to "second-guess your first superficial choice" in how a role should be played, which "prepares actors so you are a little director-proof . . . because you learn to be your own director."
Included in the long list of actors he taught were Gene Hackman, Anne Archer, Kate Hudson, Kim Cattrall, Chris Noth, Tyne Daly, Jenna Elfman, Robert Urich, Patrick Swayze, Tom Selleck and Tony Danza.
He was only 24 when he started to teach acting in New York after observing a class that failed to impress him and a friend convinced him he could do better.
"When I teach, my job is to bring out whatever is possible," Katselas said in 1998 in Buzz magazine. "It's not my job to push the ejector seat on somebody's dreams."
A 2007 New York Times profile of Katselas questioned whether Katselas' longtime study of Scientology had affected his instruction but also theorized that the tenets of the religion -- especially regarding communication -- may have enhanced his teaching.
Some students had reportedly left the playhouse because they felt pressured to join the church, according to the article.
When asked in the Buzz interview about his Scientology connection, Katselas replied: "I've learned many things in my life, and I apply them. But am I teaching Scientology? No, that's not what happens. I'm interested in art."
Joan Van Ark, an actress who appeared on the TV series "Knots Landing," enrolled in his master class about a year ago after repeatedly hearing about Katselas' skill as a teacher.
"He had a wonderful genius for perception and for seeing what was missing in a scene. He taught you how to take it to the next level," Van Ark said Tuesday. "He's just irreplaceable. As actors, we've lost our shepherd."
Milton George Katselas was born Feb. 22, 1933, in Pittsburgh to Greek immigrant parents. His family ran a small restaurant near a local electric plant, whose workers kept its 14 stools filled.
Eventually, his father "bought a movie house with a pool hall under it," Katselas told The Times in 1985. "That's where I started to do my studies in human psychology -- I did some hustling there."
In Pittsburgh, Katselas studied theater at what is now Carnegie Mellon University.
After graduating in the 1950s, he went to New York, "scared stiff" about breaking into theater, he later said, and studied with Lee Strasberg at the Actors Studio.
When Katselas spotted Elia Kazan walking down the street, he chased down the director and spoke to him in Greek.