Image copyright Casey Curry / Invision / AP Image caption Paul Dano (l) plays a young Brian Wilson (c) Brian Wilson, while John Cusack (r) plays him during the 1980s

Imagine for a moment that you're in charge of a Brian Wilson biopic.

The same Brian Wilson, now aged 73, who co-founded the Beach Boys, created iconic songs including Good Vibrations and God Only Knows using innovative and original production techniques, but who also took a lot of drugs, spent a fabled three years in bed, and put a sandpit under the piano in his lounge.

Where would you start?

By questioning the terminology if you're Bill Pohlad, who directed Love & Mercy, the new film based on the life of Brian Wilson.

"I always shy away from the biopic reference," explains Pohlad, who produced 12 Years a Slave and Brokeback Mountain.

"That may not be fair but biopics are trying to tell the full cradle to grave story of a person's life.

"When you've got somebody like Brian Wilson, who's had such an amazing life, you end up focusing so much on the celebrity parts or the bits that we've heard about, that there's not enough time to get intimate with the character and that really isn't that interesting to me".

So Pohlad chose to pull out two threads from Wilson's life, which he then intertwines to make this film.

One strand chronicles how Brian Wilson coped with the Beach Boys' first success, the making of their 1966 album Pet Sounds and the early decline in his mental health.

The other strand shows Wilson in the 1980s when he was being manipulated by controversial therapist Eugene Landy, and details Wilson's relationship with Melinda Ledbetter, who would become his second wife.

'Gut thing'

Paul Dano plays the 1960s Wilson, with John Cusack taking over the character for the 1980s alongside Elizabeth Banks as Melinda and Paul Giamatti as Eugene Landy.

"When I cast Paul (Dano) I didn't know if he could sing or not," admits director Bill Pohlad.

Image copyright Mark Davis / Getty Images Image caption Melinda Wilson, Brian Wilson and their daughter Carnie Wilson link arms attended the the LA premiere of Love & Mercy together

"It was just that he was the guy to play Brian Wilson, it was a gut thing.

"Then a couple of weeks later we had one of Brian's vocal guys go to New York and meet with Paul and 45 minutes after their meeting we got this excited text video of Paul singing God Only Knows for the first time the whole way through, and it was brilliant".

This fortuitous turn of events meant that Pohlad and Atticus Ross (who was in charge of the music), were able to use Dano's voice in the film for scenes showcasing how Pet Sounds came to life.

It still presented a conundrum though, knowing how familiar the original Beach Boys' tracks would be for much of the audience watching.

Pohlad faced the cinematic choice of getting his actors to record new vocals, like Joaquin Phoenix did for the Johnny Cash biopic Walk the Line, or have the actors mime, like James Corden did when Britain's Got Talent winner Paul Pott's voice came out of Corden's mouth in One Chance.

It all affects the believability of the performance, and in the end Pohlad went for a mix.

"In the film Paul (Dano) sings God Only Knows and Surf's Up totally unassisted," he says.

"But in other elements we show the recording process and we start with Paul's voice and then blend Brian's voice in and the rest of the Beach Boys in to complete it with the original vocal tracks".

Image copyright PA Image caption Brian Wilson continues to perform live

It's worth noting that John Cusack does not sing as part of his portrayal of the older Brian Wilson.

While a script about Wilson had been brewing for some time, Bill Pohlad still needed to earn the trust of Brian and his wife Melinda.

'No ego'

"I think the first question he asked me when I first met him was, 'so who's going to play me?' but he's not like that at all," remembers Pohlad.

"It was a joke because he doesn't have an ego about these things".

The couple were consulted when the script was being written and also once the film went into production, but Pohlad was very much in charge of the creative and editorial vision.

"He was around for us," says Pohlad "and he would dip in at different times when we needed guidance to say what really happened but otherwise they weren't all over us".

Over the course of two hours on film, Pohlad does address some of the myths and rumours that continue to swirl around Brian Wilson.

Such as, did hearing The Beatles' Sgt Pepper really cause Wilson to abandon Smile, the album that was to come after Pet Sounds, and instead head to bed for 1,000 days?

"There was a much bigger story to that," says Bill Pohlad.

"It really was a friendly rivalry but very distinct.

"We reference in the movie that Brian did hear Rubber Soul and it impressed him the way it was put together, and that pushed him to do Pet Sounds.

"And then I think someone from Brian's camp took a tape over of some of Pet Sounds and Paul (McCartney) and John (Lennon) heard it and it was like 'Oh' so they stepped up their game, and that's what sparked Sgt Pepper to a large degree.

"And then Brian was going to answer it again with Smile but unfortunately that was when he started experiencing more issues with his mental health as well as some drug abuse issues and the whole Smile project collapsed over a number of years."

Solo screening

Brian Wilson Presents Smile was eventually released in 2004 following a series of sold out live performances in London.

As for Love & Mercy, Brian Wilson watched the movie alone at his request before it was shown at the Toronto Film Festival in September 2014.

Image copyright Francois Duhamel / Roadside Attractions Image caption Paul Giamatti plays controversial therapist Eugene Landy, while Elizabeth Banks plays Wilson's wife Melinda

"That was his decision and I respected that," says Pohlad.

"I heard from the projectionist that he came out saying he really liked it".

Pohlad and Wilson later saw the film together, but for Wilson's wife Melinda, the initial reaction was a little different.

"I sat with Melinda when she first saw it, before Brian saw it and she was shocked, stunned, I think she didn't like it, so I was a little bit mortified," Pohlad recalls.

"She later told me the story that she had to drive around for two hours by herself and just put it in some context and then she saw it again with other people and everybody loved it, so she loved it and it was easier to accept at that point."

It may be very early to whisper the word "Oscar" but I just did and I'm not the first to do so either.

Bill Pohlad concedes while "you don't want to be doing it for that reason" he remains proud of the team he assembled for Love & Mercy and concedes if Oscar nominations followed next January, "that would be great honestly if that happened".

Love & Mercy is released in cinemas across the UK on 10 July.

Natalie Jamieson's interview with Bill Pohlad will be featured on Claudia Winkleman's Radio 2 Arts Show this Friday between 22:00 and 00:00.