Stop to consider the movie and TV characters that are most permanently seared into the American psyche, and their impact is rarely a function of screen time. Usually, the effect on audiences is immediate: Think Tim Curry’s first appearance in “The Rocky Horror Picture Show,” or Stockard Channing breezing into Rydell High alongside her fellow Pink Ladies.

Whether they were memorable because of their abrasiveness (Danny DeVito in “Taxi”), their rebellious streak (Ms. Channing in “Grease”) or their ability to solve a crisis with a slice of cheesecake (the titular golden girls of “The Golden Girls”), every actor who eventually went on to make Hollywood history first had to clear the hurdle of a casting department. And for many of the biggest movies and TV shows of the last half century, Joel Thurm was a central part of those teams, handpicking the actors whose performances would resonate for decades to come.

In his newly released memoir, “Sex, Drugs & Pilot Season: Confessions of a Casting Director,” Mr. Thurm, 80, details what he saw in stars like John Travolta, whom he cast in “The Boy in the Plastic Bubble.”

“I knew he wasn’t Vinnie Barbarino,” Mr. Thurm said of managing to look past the actor’s biggest role to date, on the ABC sitcom “Welcome Back, Kotter.”

Being able to spot the je ne sais quoi that many refer to as star quality is a skill, one that Mr. Thurm has capitalized on throughout his 35-year career.

“The best example I have is when someone walks into a room and has something special that you haven’t seen in other people,” Mr. Thurm said in an interview this week. “Are they astoundingly beautiful? Are they so incredibly good-looking? They could be bad-looking! It’s individual; you can’t really explain it.”

That “it” factor is the common denominator among all the stars who go on to become household names, according to Mr. Thurm, who said he had seen it immediately in Farrah Fawcett when she auditioned for the role of a stewardess on “The Bob Newhart Show.” She didn’t get the part, but Mr. Thurm said he had known “there was something special about her.” He also instantly saw it in a 17-year-old John Travolta when he met him in New York.

“He had a presence, and you can feel it,” Mr. Thurm said. “They had that little extra something.”

At the time, Mr. Travolta was most popular for his role on “Welcome Back, Kotter,” and producers would not move ahead with “The Boy in the Plastic Bubble,” a TV movie, unless a big star signed on to the project, Mr. Thurm said. He spent a lot of time with Mr. Travolta’s manager sitting on his “back deck getting melanoma and reading scripts,” Mr. Thurm said. When the script came up, they both lobbied Mr. Travolta, who agreed to sign on. Mr. Thurm later cast Mr. Travolta in “Grease,” and the rest is Hollywood history.

Mr. Thurm, who retired from a full-time casting position with NBC in 1990, hasn’t kept especially close tabs on the stars of today, but he does know enough to recognize that they tend to skew young.

“They’re all 12-year-olds,” he said. “I have only seen them once they are already stars. Ariana Grande, she’s already a star.”

Whether or not star quality has changed since Mr. Thurm started his career, Hollywood itself certainly has. In addition to snippets of back-room scenes detailing how some of TV’s most beloved characters came to appear on some of America’s favorite sitcoms, “Sex, Drugs & Pilot Season” is also filled with personal anecdotes that would — at minimum — raise eyebrows in a world reshaped by the #MeToo movement.

As a gay man living in Hollywood in the 1970s, ’80s and ’90s, Mr. Thurm often found himself in situations that almost certainly wouldn’t fly today — like massaging the actor Robert Reed’s back after he had to undergo several hair treatments for his role “The Boy in The Plastic Bubble.”

“I started to rub his back, then I rubbed, you know, started rubbing a little lower,” Mr. Thurm said of Mr. Reed, best known for playing Mike Brady in “The Brady Bunch.” “He was just miserable on the set because he was not used to not being the center of attention.”

In his memoir, Mr. Thurm also details an encounter with his teenage idol, Rock Hudson. At a party with other gay men in Hollywood, Mr. Hudson motioned to Mr. Thurm to follow him to a room upstairs.

“I was so anxious and nervous that my body below the waist could not cooperate,” Mr. Thurm wrote.

It was a moment he has never forgotten.

“I saw every single movie that he ever did and so even to find myself at that party, I thought was amazing,” Mr. Thurm said. “This is my introduction to Hollywood.”

Besides detailing his sexcapades, Mr. Thurm also takes full accountability for “the damage you may have suffered while watching David Hasselhoff,” he wrote. He initially cast Mr. Hasselhoff as Snapper Foster on “The Young and the Restless” in 1975. He later cast him in “Knight Rider” — a high-water mark in what he described as an era of “junk TV” — after a contentious standoff with producers, who originally wanted Laurence Olivier. (“Yes, David Hasselhoff and Laurence Olivier on the same list,” he wrote.)

The memoir is not just about Mr. Thurm’s dealings in Hollywood but his upbringing: growing up on a kosher milk farm in East New York. Attending Hunter College in Manhattan when it was nearly an all-girls school. Hanging out in Greenwich Village in its bohemian heyday. Flunking out of college and traveling through Italy in his early 20s.

“To me, it was just my experiences — you know, growth going through life and growing up,” Mr. Thurm said. “I have no regrets. Nobody died.”