In Michael Bay’s new action thriller “Ambulance,” Jake Gyllenhaal plays career criminal Danny Sharp. One of the highlights of the film is an intense stunt where Gyllenhaal hangs outside the door of the titular vehicle, shooting a gun while helicopters chase him, his adopted brother Will (Yahya Abdul-Mateen II) and their hostages — EMT Cam Thompson (Eiza González) and a patient — as they speed along the bank of the Los Angeles River.

At the film’s Los Angeles premiere on Monday night, Gyllenhaal revealed that the scene wasn’t in the original script. Instead, Bay came up with the idea the night of filming.

“I had no idea,” Gyllenhaal told Variety on the red carpet outside the Academy Museum of Motion Pictures. “We were on our way home, Yahya was driving the ambulance. We were almost finished with our day, and we were driving to get back into our civilian clothes, out of our character’s clothes. Michael called us on the radio and was like, ‘Wait, come back! They opened up the L.A. River!'”

When the actors arrived back on location, Bay shared his plan. “‘He was like, ‘I have an idea! I have two helicopters!'” Gyllenhaal recalled. “And I was like, ‘How? What are you talking about, you have two helicopters? Where are they?'”

Once Bay got the crew on the same page, they planned out the shoot through a mix of improvisation and coordinating with the experienced helicopter pilots that were hired for the stunt.

“[Michael] just planned the whole scene, sort of an improvisation of the scene, and it went that way,” Gyllenhaal explained. “Every day was a little like that. And I was like, ‘That’s why you make a Michael Bay movie.’ That’s the reason. To hang out of a moving vehicle and have fun.'”

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Jake Gyllenhaal as Danny Sharp in “Ambulance.” Universal Pictures

Bay confirmed the last-minute nature of the shoot, noting that the whole of “Ambulance” was filmed extremely quickly, wrapping production after just 38 days.

“That scene was written on a Sunday,” Bay noted. “I write my own action, and I’m a very fast writer doing that. I go, ‘I need a helicopter scene.’ I wrote this for the best helicopter pilots in the world. And we shot that scene in two and a half hours. … I’m very fast at coordinating action.”

During the scene, the helicopters fly extremely close to Gyllenhaal. But in spite of the challenges of the stunt, Bay said Gyllenhaal was enthusiastic about performing and handled it like a pro.

“I said ‘Listen Jake, the helicopter’s way up there, its gonna come down about 20 feet from you. These are the best pilots in the world, these guys have flown for 25 years. It might scare you, but don’t be scared,'” Bay recalled. “‘And he loved it. He thought it was great.”

Abdul-Mateen II experienced the sequence from the relative safety of the driver’s seat, telling Variety about the challenges and the rewards of improvising action scenes for the film.

“This was a very exciting movie and the making of this movie was very much like the experience of watching it, it was all hands on deck” Abdul-Mateen II said. “The game was basically to say, ‘Yes and…’ And the entire job was to keep the ball in the air. So I had a great time.”

As a result of all the stunts that the actors did on set, though, many walked away a little beat up.

“I had a lot of injuries,” González shared. “More than injuries — bruises, a lot of bruises. I was covered in bruises all the time. But, you know, par for the course.”

The atmosphere at the premiere was similarly lifelike, with a replica of the film’s ambulance parked on the carpet, flashing its lights and sirens as the movie stars and guests, including Jason Momoa (who co-stars with Abdul-Mateen II in the upcoming “Aquaman and the Lost Kingdom”) walked into the theater.

“Ambulance” opens in theaters April 8.