Chemistry is a vital ingredient in any romcom. But what happens when actors who've barely met in real life suddenly need to convince audiences that they're old friends – who not only go way back but know each other intimately?

Reese Witherspoon admits she “had a little panic attack” about this very thing before she started shooting scenes with Ashton Kutcher for their new Netflix movie, Your Place Or Mine.

The film sees the pair play Debbie (a single mother in Los Angeles) and Peter (a brand consultant in New York) who, despite having very different lifestyles, became good friends following a one-night stand two decades earlier.

Debbie and Peter keep in touch via phone – which meant Witherspoon and Kutcher's early scenes together were mostly split-screen conversations, with each of their parts shot separately. So they couldn't even spark off each other in the same room, hence Witherspoon's panic about conjuring up some convincing chemistry.

“Because really, we didn't know each other,” says the Legally Blonde star, 46, who had only “fully met” her co-star once before at a party. “I was like, I don't know this guy, and we're supposed to be best friends for 20 years.”

So, how did they pull it off?

“Probably about a month before, we started sending each other videos,” reveals Witherspoon, who agrees not having much physical time together made things more challenging.

“Every day,” Kutcher, 45, chips in. “We would send back and forth videos: How's your day going? What's going on? What's happening with you? What are you afraid of? Like wait, hold on – if you had pancakes, would it be syrup?”

Witherspoon interjects: “Who's your best friend? And like, why are they your best friend?”

Things “got real personal”, Kutcher notes.

“And it was good,” says Witherspoon. “It was really, actually kind of fun. And then our kids would get in the videos, and our dogs. You met my dad on one,” she adds, recalling the time they all “hung out” on a video call on her porch.

The method worked. As Kutcher concludes: “By the time we started shooting, we were sort of used to communicating this way, so it felt really natural.”

The plot sees Peter and Debbie swap homes – and lives – for a week, when he offers to travel over and look after her tween son Jack, so she can have a break in New York.

The experience opens their eyes to the fact that maybe there's unfinished business between them after all – but not until after Debbie has some fun on the town with Peter's neighbour Minka (played by Love Life's Zoe Chao), as well as a steamy fling with literary editor Theo (Jesse Williams).

Your Place Or Mine is the creation of writer Aline Brosh McKenna, who also makes her feature directorial debut with the movie.

The 55-year-old already has a stellar list of writing credits under her belt – including The Devil Wears Prada and 27 Dresses, as well as co-creating the hit comedy series Crazy Ex-Girlfriend with the show's star Rachel Bloom (who also has a role in Your Place Or Mine).

Why did she decide to make the leap into directing with this project, as well as writing the screenplay?

“The story was based on personal experience,” Brosh McKenna explains. “And I wanted to do a romantic comedy for people who have been through something in their lives and have accumulated some life experience.”

“As I was working on that, it just felt very personal to me through the whole process,” she adds. “It was an idea I'd had for a long time.”

There are plenty of classic romcom hallmarks. But Brosh McKenna was keen to avoid some of the more limiting character cliches – especially in terms of letting Debbie embrace her status as a single woman with a fully formed life of her own.

“[Debbie] has three men who are interested in her in this movie,” she elaborates. “One day, Reese was like, ‘Why did they all like me, they all really like me?!'

“But it was important. I wanted to show, when you're falling in love and you're not 21, that there's maybe – she has more of a romantic history, and there's other people in her life.

“Sometimes romantic comedies, the female lead is incredibly chaste, to a point which seems unrealistic. And so that was fun, to give her a number of different love interests to play off of.”

Brosh McKenna also co-wrote a song, Embers, which features throughout the movie and was specially created to capture Debbie's story and the central thread of her relationship with Peter.

“We wanted a song that was just hers,” Brosh McKenna explains. “I was lucky enough that Sid [Siddhartha Khosla] who did the composing and Alan [DeMoss] who works with him, invited me in to write a song with them.

“It was really fun and we spent hours on the phone, and I told them stories about why I had written the movie and some of the experiences that had led me to write it, and what my inspirations were.

“We talked a lot and came up with this idea of Embers – that it's a relationship that is not in flames at the moment, but it's still there. It just needs to be stoked.”

Your Place Or Mine is available on Netflix from Friday, February 10.