It’s a Chicago thing.

Because “Grease” was loosely based on one of the creators’ experiences at Taft High School and was first staged as a musical on Lincoln Avenue, we have to ask John Travolta for his reaction to the fan theory sweeping the Internet about the movie version: the idea Sandy (Olivia Newton-John) is in a coma for pretty much the entire film and then dies and goes to heaven with Danny (Travolta) in the final scene.

“Yes, I’ve heard about that,” Travolta says with a chuckle.

He thinks it’s fantastic that fans use their imaginations like that and that it’s a tribute to the longevity of the film, released 40 years ago this month.

If certain fans want to believe in Sandy’s eternal sleep, the star says: Have at it.

Travolta and his wife Kelly Preston were in Chicago last week to promote “Gotti,” starring Travolta as the Gambino crime family boss John Gotti and Preston as his wife Victoria. I hosted a conversation with the two after a screening Tuesday night and sat down with Travolta again on Wednesday, and we will soon get to the meat of the matter.

First, one other quick Chicago thing.

Travolta’s breakout film, “Saturday Night Fever,” struck a chord with the late, great Gene Siskel in a way few films did. In 1978, Siskel purchased Travolta’s famous white suit from the film for $2,000. Seventeen years later, it sold at Christie’s for $145,000.

The white suit that John Travolta wore in "Saturday Night Fever" (alongside Karen Lynn Gorney) belonged for a time to Chicago film critic Gene Siskel.

The white suit that John Travolta wore in “Saturday Night Fever” (alongside Karen Lynn Gorney) belonged for a time to Chicago film critic Gene Siskel. | Paramount Pictures

“Gene had it, and Jane Fonda [tried to get] it, but I never had it,” says a laughing Travolta. “I think Gene paid for a vacation home with the money he made off the suit.”

From Tony Manero in “Fever” to Danny Zuko in “Grease” to Bud in “Urban Cowboy” to Vincent Vega in “Pulp Fiction,” Travolta has played some of the most memorable characters of the last half-century.

Now, finally, comes the long-delayed “Gotti,” which hits theaters June 15 — more than seven years after Travolta joined the project.

At one point, Barry Levinson (“Diner,” “Rain Man”) was on board to direct. Al Pacino and Joe Pesci were reportedly going to co-star.

None made it to the first day of filming.

The only constant throughout the process was Travolta.

“In this day and age, human stories or historical stories are very difficult to get made. I’m proud that not only did we get it made, but we got it released properly, in theaters. …

“There were so many invitations for me to say goodbye to it … but when I had lunch with John Gotti Jr. [in 2010] … 300 photographers showed up at the window of the restaurant. … I thought, ‘There’s an interest in this. They like the idea of John Jr. and I being together.’ Why? What is going on here?

“So I so deeply felt like … this was something worthy of sticking to, and with.”

Directed by Kevin Connolly (best known for playing “E” on “Entourage”), “Gotti” is based on John Gotti Jr.’s book “Shadow of My Father,” which chronicles the son’s relationship with the ruthless gangster known as “The Dapper Don” and “The Teflon Don,” who spent his last years incarcerated at the United States penitentiary in downstate Marion before he died of cancer in 2002 at the age of 61 in a prison medical facility in Springfield, Missouri.

Says Kelly Preston: “We went to [Victoria’s] house, and she cooked us an amazing meal … and we could ask her anything. I became email buddies with her and could ask her, ‘How were you feeling during this moment?’ ”

Kelly Preston plays Victoria Gotti in "Gotti."

Kelly Preston plays Victoria Gotti in “Gotti.” | Vertical Releasing

Travolta: “They let us borrow the personal effects. I got to wear his coat. It still had the cologne … jewelry, handkerchiefs — all those little bits, believe it or not, really contribute to the building of a character.

“My mother taught acting, and part of it was building … putting layers on it … and one moment you arrive, when all your ingredients are there, and you kind of know when you have it. All those little things help.”

Though Travolta has played some of the most beloved characters in recent film history, he’s also been the villain on many an occasion. And although “Gotti” features scenes of the mobster looking after the neighborhood and being the loving husband and father, there’s never any doubt Travolta is playing one of his most despicable characters ever.

“The audiences are so comfortable with me playing a bad guy, whether it be ‘Face/Off’ and, well, Vincent Vega wasn’t really a bad guy, but he was up to no good. … There’s some pleasure that I think they have in me in particular playing the darker side of characters. …

“I’m all about, ‘Give me something to do.’ I don’t want to show up as just me. It’s fine, but I really enjoy … when I did ‘The People vs. O.J.,’ I loved playing Shapiro because it gave me something to do, someone else to be. … I’ve never viewed myself as a leading man. I’ve viewed myself as a character actor.”

Married for 26 years, Travolta and Preston have worked together before, from “The Experts” to the infamous “Battlefield Earth” to “Old Dogs” — but it’s a relatively rare thing for them.

“I’m very sensitive to that,” says Travolta, “because I know often in Hollywood they’re not big fans of couples working together. … I’m very careful about when it’s OK to do that. When I feel that an actor as good as Kelly would be cast in that part regardless if I were involved or not, then that’s the correct time. … If you have that certainty, you are in safer territory.”

John Travolta and wife Kelly Preston promote "Gotti" at the 2018 Cannes International Film Festival.

John Travolta and Kelly Preston promote “Gotti” at the Cannes International Film Festival. | Getty Images

There’s a sequence in “Gotti” in which John and Victoria’s son Frankie is killed in a car accident. Travolta and Preston’s son Jett died suddenly in 2009 at the age of 16.

Says Travolta: “Although we both had experienced the worst tragedy a parent could possibly experience, that’s OUR experience. … What was John Gotti’s experience, what was Victoria’s experience? You have to look through their eyes. How was it different?

“John Gotti Sr. wouldn’t dare cry in front of others. I would cry if I’m feeling that. I would cry openly. He would cry hidden in the living room when everyone is asleep. …

“You have to really become someone else and view that. Otherwise, you’re just playing yourself, which may not always be the right note to play. …

“I don’t like cross-collateralization. I think your art is your art, and your personal life, if you need to inform by it, that’s fine … But I don’t know if whether feels right to borrow from [one’s personal life].”

Kelly Preston: “It’s definitely tricky. For me, it was hard to navigate to NOT go there. I would try to keep it separate … but you’re feeling the same depth of the tragedy.”

At the classic Chicago diner Club Lucky in Wicker Park, I mention to Travolta that some of his peers have recently revisited iconic roles, like Harrison Ford with Han Solo, Tom Cruise in the upcoming “Top Gun: Maverick.” Which of his past characters would he like to revisit?

“I always thought the ‘From Paris with Love’ character” — maverick CIA agent Charlie Wax. “It would be fun to see what happened to him.

“I did the sequel to ‘Get Shorty’ called ‘Be Cool,’ so I got to revisit that. And [we got to see] Tony Manero with ‘Staying Alive.’

“A prequel to ‘Pulp Fiction’ would be awesome. But … Quentin is very particular, and only he would be the one to allow that to happen. No coercing. … In my opinion, it’s his crown jewel, ‘Pulp Fiction,’ and maybe he doesn’t want to touch it.”