Marilu Henner Initially Thought Grease Isn't 'Gonna Be a Hit' on Broadway: 'I Made a Mistake'

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Long before it became a classic, Marilu Henner appeared in the very first production of Grease — but she wasn't there for its opening night on Broadway!

The actress, 70, tells PEOPLE that even though she played Marty in the original Chicago company of the musical, she didn't have faith Grease would last long if and when it transferred to the Great White Way.

"I was just starting at the University of Chicago, and I thought, 'No way is this thing gonna be hit. I'm not gonna waste the time leaving school,' " she explains, adding that the 1971 Chicago production was "gritty" and included "37 songs in the first act alone."

In fact, she adds, "My mother wouldn't come to see it because I said 'F you' in it seven times. She said, 'Mari, I don't care if you take off your clothes, I just don't wanna hear you swear.' "

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However, when the musical opened on Broadway in 1972 — with Katie Hanley in the role of Marty — "I could not believe how much they changed it," says Henner, while celebrating the release of the new book Grease, Tell Me More, Tell Me More.

"I saw it on a class trip and I was like, 'Damn, I made a mistake. This is so fabulous,' " she adds. "So when the opportunity came up for me to do the national company, I did it."

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Henner toured with the musical alongside John Travolta — who played Doody in the stage version before eventually starring as Danny Zuko in the 1978 film — as well as her Taxi costar Jeff Conaway. The three would eventually do the show on Broadway as well.

"I first met [Jeff] doing Grease because he was Danny Zuko," she says of the late actor, who died in 2011. "He was like my brother."

Henner adds that she and Travolta, now 68, were the "two youngest" in the cast around ages 18 and 19 when they were on tour.

"We were great friends, and we were friends for a long time," she says of Travolta, whom she also dated on and off at the time.

While traveling with the musical, Henner says there were "romances, accidents, dramas, cliques" and even accidents.

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"Are you kidding me?!" she says, recalling a time she was running late to a performance of Grease in Michigan.

"Johnny and I were hanging out in Detroit, and all of a sudden, it was like, 'We're late, we're late. We're late.' This was May 6th, a Sunday," says Henner, who has hyperthymesia or total recall memory.

"We like ran through the door, and I left my hand behind me. And by the time we got to the theater, the bone was like almost sticking through! I was like, 'I think I broke my hand.' The stage manager said, 'We're going to the hospital.' So then I had them put a half cast on me so I could still get dressed and do the show and everything."

Despite the injury, she adds, "You don't care because you're young, you're happy ... That road life is very special."