'I worried about that': Laura Dern admits she 'was concerned' about working opposite Will Arnett in new movie

EXCLUSIVE: Laura Dern has worked on all kinds of movies, from blockbusters like Jurassic Park to emotional dramas such as Marriage Story.

But the Oscar winner tells nine.com.au working on Bradley Cooper's latest directorial offering, Is This Thing On?, had an unexpected element that left her a little "concerned".

Dern plays retired volleyballer Tess Novak opposite funnyman Will Arnett, who stars as her husband Alex Novak – a man who tries stand-up comedy as their marriage falls apart.

Arnett co-wrote the movie – loosely based on UK comedian John Bishop's life – together with British writer Mark Chappell, before Cooper got involved and re-worked the script.

Cooper then asked Dern for her input, so when the 58-year-old actress went to film scenes opposite the original writer, she admits she was nervous.

"I worried about that," she tells nine.com.au in London.

"I think I was concerned because [Arnett and Chappell] had an idea of Tess ... then Bradley invited me in to sort of bring what I was seeing in Tess.

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"He and I collaborated on this, including [Tess] as a professional athlete and John and Mel Bishop and their relationship, which inspired this, and I spent time with Mel, so it became this morphing of all this energy.

"I hoped Will would feel great in the discovery but I think at the very beginning, I worried."

It turns out Dern had nothing to be scared about because instead, she believes it was "amazing" and added another layer to the movie.

"[It] added to the idea of, 'I thought I knew you, but there's so much I don't know'," she says.

Dern has known Cooper for two decades and he, in turn, has been best friends with Arnett for more than 20 years, playing the common link between the two actors.

While Dern and Arnett had "met socially a few times" and would "riff and make each other laugh at something, do something silly", they didn't know each other very well.

So, Cooper instructed them through a "relationship bootcamp" of sorts to get to know each other better and make their long marriage on-screen more believable.

"He [Cooper] immediately led us to dive so deep you would not believe it, before we started," she recalls.

"We did a very personal workshop, the three of us, for a week together before we even started prep and that was amazing."

Cooper also "demanded" another type of bootcamp from Dern – getting her to train like an athlete to understand Tess's mindset.

"Bradley also entrusted me, I guess (and demanded), the discipline of feeling her story, feeling it in my body," Dern says.

"So I was also training constantly and feeling the rigour of that kind of discipline, just energetically really helped."

Dern's commitment to the character is something that shone through to co-star Andra Day, who plays her best friend in the movie, Christine.

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"I realised she puts so much work into understanding every single nuance about her character," Day tells nine.com.au.

"I think that's what makes her such a joy to be around, I feel like she really delves into the nuances of herself."

Day says there was an "ease of dropping into being besties" with Dern that "floored [her] a little bit", as the Grammy-winner shared the set with the Oscar-winner.

"She was so wonderful... she was just so generous and so willing and vulnerable," she says, gushing just as a real-life best friend would.

"Anytime they yelled, 'Cut!', everyone on set just sort of gravitated toward her, just to receive any type of wisdom."

The other person Day has enormous admiration for is her on-screen husband, Cooper, calling the filmmaker a "visionary".

"I tell people: there's talented people, there's hard working people, there's passionate people, but when all of those things are in the mix, then you have a visionary and that's what he is," Day says of the A Star Is Born director.

"So when he called about this role, I'm like, 'I don't actually care what it is, I definitely want to be a part of this'."

Is This Thing On? is out in cinemas across Australia now.

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