Oscar-winning director Susanne Bier suffered "sleepless nights" after Sandra Bullock refused to cut holes in the blindfold she wore for their horror thriller film, Bird Box.
Bullock plays a lone mother with two young children, battling an unseen force which compels you to kill yourself if you see it.
Hence the need to constantly wear a blindfold.
The post-apocalyptic film sees Bullock, who plays Malorie, guiding the children down a treacherous river and through a dense forest, all without being able to see a thing. All of this can be seen in the film's trailer, so don't worry, there aren't any spoilers to follow.
But the chances of Bullock disappearing over the side of a boat or into a boggy ditch were definitely higher than Bier would have liked.
She says: "I had a completely dedicated, crazily diligent actor - I said 'can we just make holes in the blindfold?' and she said 'no way, no way'.
"I kept hoping she wouldn't bump into the camera."
Bullock was assisted by an expert who "helps blind people navigate spaces", leaving Bier relieved that the actress "acquired quite sophisticated techniques" for moving around.
Not surprisingly, the Oscar-winning star did still "fall over a number of times", and Bier smiles: "I'm relieved she's still looking good."
Bullock joked as she told Deadline: "It made her very happy when I was blindfolded running into the camera."
But the actress said the blindfold didn't just make moving around difficult.
"What's the easiest way as an actor to show how you're feeling on camera? It's your eyes. And you realise when that's taken away you don't know how to act."
Speaking to Reuters, she added: "But I think in the end... it helped give a really jagged feeling to those scenes rather than if I had holes cut in my blindfold and I could see and I was pretending to stumble and be blind."
This film, which depicts Malorie's transformation from reluctant parent into a machete-wielding mother bear, is an unusual take on motherhood.
"It's a portrayal of a very contemporary female heroine," says Bier, who also directed BBC One's hugely successful drama The Night Manager.
"It's a female narrative - defined by women and not men - and is very unconventional in a thrilling, scary movie.
"It's a depiction of motherhood which doesn't fall into the conventions and cliches we've seen in hundreds of films."
Bier relished "describing motherhood within the context of something mainstream".
Sarah Connor from the Terminator films is another action-packed mainstream maternal film heroine, who's often described as going from "timid" to a "hardened warrior".
But the majority of action heroines are childless, or if they do have children you don't see very much of them. This film focuses on Malorie's journey through motherhood, with the children sharing centre stage.
Having been reluctantly pregnant at the start of the film, the story is as much about her transformation as a parent as it is about the external terror she's battling.
She has to frighten the children by shouting "if you look, you will die" at them, in order to keep them safe.
Bier says it was "hard on Sandra being harsh to them".
"All her character wanted to do was have those kids survive. And she does that by whatever means possible, and because of that you accept her harshness.
"Never at any point do you not understand her or dislike her - I'm hoping it's fascinating to watch."
Bird Box is based on Josh Malerman's best-selling novel of the same name, which is billed as a "Hitchcockesque psychological horror".
Given you don't actually see the monster ravaging the planet's population, much of the tension comes from people's gruesome response when they encounter it.
But it seems that some critics love to see a monster in the flesh, with one saying they felt short-changed on thrills.
Variety wrote: "In the endless debate of how much creature to feature in a horror movie, Susanne Bier's supernatural thriller unwisely withholds the monster altogether."
Bier, who won an Oscar for 2010 thriller In A Better World and an Emmy for The Night Manager, responds robustly to this, saying "for me it's a huge strength" not to see the monster.
"I find it more scary before the monster appears, there aren't any films where I didn't get slightly disappointed once I'd seen it," she adds.
"I wanted to make a movie which had that tension the entire time. I think whatever those beings are, they tap into your mind and it messes with it never to see them."
Bier wants to 'engage emotionally'
The Guardian said "the what, why and how of the crisis never gets answered" but IndieWire described the film as "so intense you'll want to cover your eyes" and Entertainment Weekly said Bier "keeps the mood taut and defiantly in the moment".
Bier's main aim with a film is to "engage emotionally", and she says while she often takes on board reviewers' comments, she's fairly detached if she really doesn't agree with a particular reviewer.
"Sometimes I read reviews and I go, 'I have nothing in common with this person'," she says.
She was able to attract a stellar cast of survivors alongside Bullock, including John Malkovich, Sarah Paulson, Tom Hollander, Trevante Rhodes and Parminder Nagra.
Some of them play the people trapped with Bullock and Moonlight star Rhodes, in a house owned by Malkovich.
It becomes a battle of wills as they work out how to eat and travel to safety.
Bier said she had to juggle a lot of big personalities.
"It was insanely vital to a point where, as a director, you are going 'okay, let's just get some order into this wonderful chaos'. You had to kind of steer it."
Beyond Bird Box, she will be directing HBO series The Undoing, starring Nicole Kidman and Hugh Grant and written by Big Little Lies' David Kelly.
And as the winner of an Oscar, Golden Globe and Emmy, work opportunities should be pretty fast-flowing for Bier.
"I can certainly choose more than I could some years ago," she says.
'Still a men's club'
But she adds: "I've never been a careerist - I will choose something intriguing - it might be successful or not successful but that's how I work."
She thinks there are now "more opportunities" for women behind the camera, but that things are still biased towards men.
"I still feel it is very much the men's club because whoever assigns projects are mainly men," she says.
"There is a language and a way of talking - I feel that young women are, at times, passed over for projects because they don't necessarily have the same vocabulary, the same way of talking, which doesn't mean they aren't equally qualified.
"I've never been pro-quotas for women, but I think they may be necessary for a time period. It's hard to change it fast enough without making a formalised radical decision."
Bird Box is released in select cinemas from 13 December and on Netflix on 21 December.