Jonah Hauer-King's trainer shares how he built lean muscle to play Prince Eric in 'The Little Mermaid'

• As Prince Eric in "The Little Mermaid," Jonah Hauer-King was meant to be a "strong sailor boy."
• He trained like a bodybuilder and ate in a calorie surplus to put on lean muscle, his trainer said.

The brief was "strong sailor boy."

But, when the coronavirus lockdown delayed the filming of "The Little Mermaid" and Jonah Hauer-King had eight bonus months to train for his role as Prince Eric, he almost became the Incredible Hulk, his trainer said.

Sana Shirvani, a strength and conditioning coach and personal trainer in the film industry, worked with the cast of the new live-action remake of the Disney classic, but spent the most time with the two leads, Halle Bailey, who played Ariel, and Hauer-King.

Unlike the mermaids, she put Hauer-King on a traditional bodybuilding style program designed to build lean muscle without gaining fat.

"We got him to that point, and then obviously if he continued training in that capacity, he was only going to get bigger and bigger," Shirvani told Insider in an exclusive interview.

When the coronavirus pandemic delayed filming, Hauer-King started getting too muscular for the role as he continued working out, so they reduced his food intake from a calorie surplus to maintenance and tapered down his training.

"I remember the director coming up to me and saying, 'We are not after the Incredible Hulk, we want a strong sailor boy,'" said Shirvani, who trains clients one-to-one at "wellness optimization facilities" Until in London and via her soon-to-launch app.

Hauer-King was disciplined with his diet

Shirvani trained Bailey and Hauer-King for three months before filming was set to commence in March 2020. But just three days before shooting was supposed to begin, production was shut down due to the coronavirus pandemic. With much of the world in lockdown, it was unclear for how long the stars would have to maintain their strength and physiques.

Filming didn't start for eight months, but not only did Hauer-King stay on track thanks to virtual sessions with Shirvani, he surpassed what was asked of him and put on too much muscle, Shirvani said.

Shirvani calculated Hauer-King's calories and macros, putting him in a calorie surplus with plenty of protein, and they used a meal delivery service to ensure he hit his targets. At the start, he found it hard to eat more than he was used to, Shirvani said, but the more he trained, the more he felt like he needed the food.

Hauer-King ended up eating prepped meals throughout most of the coronavirus lockdown, and Shirvani said he was very disciplined.

"He was so, so dedicated," she said. "His professionalism was so good that he just said yes to everything I threw his way."

But by the time the cast was shooting their final scenes in Sardinia, Italy, Shirvani encouraged everyone to relax their diets and "have whatever they wanted," she said.

Hauer-King trained like a bodybuilder

In pre-production, when Shirvani could train the actors in person, they would work out for 60 to 90 minutes, five to six days a week, she said.

Hauer-King trained like a bodybuilder by doing body-part splits, meaning he focused on different muscle groups each day.

"It was a standard bodybuilding program, so we would do, for example, back and biceps, chest and triceps," Shirvani said. "We would focus solely on getting a little bit of strength, a little bit of size into his body."

They employed progressive overload by increasing the weights or reps as much as possible to stimulate muscle growth, and did regular check-ins to assess Hauer-King's body composition, Shirvani said.

"He still was pretty new to gym training, so getting him strong was one of our top priorities," Shirvani said. When someone starts strength training seriously for the first time, they are able to make faster progress than seasoned lifters — this is known as "newbie gains."

Safety was important too though: Hauer-King had had ACL surgery 18 months prior, so they focused on rebuilding the strength in his knee so he was able to perform many of his own stunts.

"He was so into the training," Shirvani said. "He really enjoyed, the strength training, the progressive overload, seeing the developments."