Rock biopics are usually about exaggerating the myth behind the legend. But even though the new Brian Wilson flick “Love & Mercy,” which opens Friday, devotes plenty of time to the tyrannical reign of Dr. Eugene Landy over much of the Beach Boy’s life, the largely unknown true story of his overmedication, psychological abuse, financial control and virtual imprisonment of Wilson is even more horrific.

“After I first saw the film, I had to just drive around for a couple of hours to clear my head,” Wilson’s 68-year-old wife, Melinda Ledbetter (played in the film by Elizabeth Banks), tells The Post. “Then I remembered that what Landy did to Brian was even worse. You don’t get a sense of it in the movie, but it happened on a daily basis, for years.”

The Bill Pohlad-directed film depicts Wilson in two different narratives; in the first, Paul Dano portrays him at his creative peak in the 1960s, as he steered the Beach Boys to artistic and commercial success with hit songs, including “Good Vibrations,” and albums such as the visionary “Pet Sounds.”

The second features John Cusack as a timid, childlike Wilson during the 1980s, while Landy (played by Paul Giamatti) and a crew of flunkies monitored Wilson’s every move and controlled his business affairs, relationships and contact with the outside world.

Born in 1934, Landy had aspired to a life in show business during the 1960s (briefly managing George Benson and producing a Frankie Avalon single) before picking up a doctorate in psychology and setting himself up as a therapist to the stars, like Alice Cooper and Rod Steiger.

By 1975, Landy was hired by Wilson’s first wife, Marilyn, to use his intensive 24-hour treatment to tackle the drug abuse, weight gain and strange behavior of the reclusive Beach Boy. On one occasion in the early 1970s, Wilson managed to freak out both Cooper and Iggy Pop at a Beverly Hills party by insisting that they sing a version of “Shortnin’ Bread” for an hour.

Once hired, Landy misdiagnosed Wilson as a paranoid schizophrenic, aggressively medicated him, and pushed him into a fitness regimen. Wilson went along with the program for fear that Landy would have him committed to an institution.

“The first couple of years, he wasn’t very friendly with me,” Wilson, now 72, tells The Post. “He could be very stern — that was pretty rough.”

After disputes over fees, Landy was fired in 1976. Wilson’s behavior spiraled downward again. Abusing alcohol and drugs, he had ballooned to more than 300 pounds by 1982, at which point he was divorced and unable to care for himself. As a result, Landy was rehired, this time by Tom Hulett (the Beach Boys’ manager at the time).

“Gene taught me how to run and swim,” Wilson says. “He taught me how to eat rice and chicken instead of steaks. He helped me lose 130 pounds.”

But Landy’s unorthodox treatment, which included the employment of assistants to watch Wilson constantly, came at a staggering price — approximately $430,000 a year.

For a while, Landy even received 25 percent of the copyright to all of Wilson’s songs and was given co-credit for “writing” work on the Beach Boys’ self-titled 1985 album. But salvation was about to arrive, in the form of Ledbetter.

“Love & Mercy” begins with a confused Wilson walking into a Cadillac showroom in LA, with Landy and assistants in tow. He buys a car from Ledbetter, who worked as a salesperson at the dealership, but during the transaction, he slips her a card that reads, “lonely, scared, frightened.”

“Landy actually came into the showroom two weeks prior to that, without Brian,” says Ledbetter. “That was the first time I met him, and I knew even then he was no good.”

Wilson and Ledbetter began dating, and she realized something awful was going on.

“It was so obvious he was being drugged. We’d get in my car to go somewhere and the first thing that would happen is that Brian would fall asleep in my lap,” she says.

“Most of the time, Landy was giving him downers to keep him out of his hair,” continues Ledbetter. “Around 1988, when Brian’s solo album came out, Brian had a lot of things to do. So Landy would give him uppers.”

Gradually, Ledbetter also began fearing for her own life, taking the precaution of recording conversations with Landy and informing her mother of her movements before attending meetings with him.

Landy used his power to insert himself into Brian’s creative process (he was co-credited as songwriter on several songs featured on Wilson’s first, self-titled solo album), and was more than happy to live the rock-star lifestyle to a higher degree than Wilson.

In a Rolling Stone article from 1988, journalist Michael Goldberg reports of an outlandishly dressed Landy (even for the 1980s) sporting a silk shirt, cowboy boots and a haircut that resembled a “modified Rod Stewart shag.” He also threw a hissy fit when he realized Warner Bros. Records (Wilson’s label at the time) had sent a town car to pick him up at LAX, instead of a stretch limo.

One of the reasons Landy was able to operate such an authoritarian regime without much pushback from his patient had to do with Wilson’s troubled relationship with his father. During the 1950s, Murry Wilson was both domineering and occasionally violent toward his sons, Carl, Dennis and Brian, who went on to become the core of the Beach Boys.

Knowing the family’s history and Wilson’s subservient attitude to Murry, Landy was able to exercise a parental level of control.

“He was like a father figure in my life,” Wilson says.

“There was a total parallel between Murry and Landy,” adds Ledbetter. “Because Brian came from such dysfunction, it was hard for him to recognize how dysfunctional the situation with Landy was.”

It was so obvious he was being drugged. We’d get in my car to go somewhere … and Brian would fall asleep in my lap. - Melinda Ledbetter on Eugene Landy’s control of her husband

By 1989, Landy, now Wilson’s legal guardian, felt his client and Ledbetter were getting too close and exercised his power by forbidding them from seeing each other.

At that point, Ledbetter heightened her attempts to free Wilson from Landy’s control by working with the Wilson family. She was helped by the discovery of a will that, in the event of Wilson’s death, awarded the vast majority of his wealth to Landy.

“It seemed to me that Brian was worth more to Landy dead than alive,” says Ledbetter.

After the Wilson family filed a lawsuit contesting the conservatorship, Landy was banned from contacting Wilson in 1992.

Wilson and Ledbetter reunited in ’92, marrying three years later.

After reappraising his mental health and diagnosing him with schizoaffective disorder, a team of UCLA doctors concluded that Landy’s prescriptions had done more damage to Wilson than Wilson had ever done to himself through his own intake.

Wilson admits watching “Love & Mercy” was difficult. “[Giamatti] was very scary as Landy — he even got his voice right.”

But remarkably, Wilson still remembers his time under Landy’s care with surprising warmth. (Landy died in 2006 of complications related to lung cancer.)

“I still feel that there was benefit,” says Wilson. “I try to overlook the bad stuff, and be thankful for what he taught me.”