When Brendan Fraser took on the role of Charlie, a father and English teacher living with severe obesity in Darren Aronofsky's The Whale, he says he knew that he had a lot of work to do.

An unflinching character study of a man running out of time, The Whale – adapted by playwright Samuel D. Hunter from his play of the same name – follows Charlie through five days in his life as he deals with congestive heart failure and comes to terms with the fact that he doesn't have long left to make peace with his life.

Oscar nominee Fraser, 54, says he had to do extensive research to ensure that his portrayal of Charlie and his situation was not only accurate, but sensitive and honest.

“Everything I needed to know about Charlie's right there in Sam Hunter's play and adapted screenplay,” says the star.

“I consulted with the Obesity Action Coalition, who are an advocacy group for people who live with obesity, and they were very helpful in many aspects of being sensitive in the character.

“They gave great notes on the authenticity of the make-up, and impressed upon us how important it was to get that right.”

Creating Charlie for The Whale required the use of extensive prosthetics, make-up and costuming, which Fraser had to learn to act in and embody without dehumanising the character.

“Charlie's not a creature or an animal, he's a man, and his body is north of 500 pounds, whatever it is, but he's a big man… his mobility is going to be influenced by that,” says Fraser, who consulted with people living with obesity and eating disorders to help him understand Charlie.

“But I think that we were in good hands knowing that the make-up was created by Adrien Morot, who created Charlie's body virtually and from that appliances were 3D-printed – pretty innovative process that he's spearheading, actually – and from that, a mould can be made and then apparatus from that could be placed on the face.”

This process is important, Fraser says, “because he had absolute control over the size of the pores, the shape of the skin anomalies, wrinkles in the – everything, which gives authenticity to Charlie on the screen”.

“You're asking an audience to suspend its disbelief in a way that might work in a theatrical production, which this was initially, but on a big screen you have to meet Charlie as a person first,” he adds.

“And then you can go on the journey with him after the first five minutes… believing that's really who he is.”

The Whale is set entirely in Charlie's small apartment, which gives the audience a feeling of how isolated life is for the protagonist.

“Charlie is not very mobile, and so the nature of the story meant that we were going to be with Charlie and see what his life was like, and try to bring audiences to experience that kind of place, of being stuck,” says director Darren Aronofsky, 53.

“And so my challenge was how to keep the audience interested, and keep them excited to keep watching – but luckily, I had Sam's incredible script and great performers. And I think watching it, it transports you to somewhere else, you get lost in the story.”

Fraser adds that he was supported by movement coach Beth Lewis to learn how to navigate the apartment as Charlie does, praising the “invaluable training and advice” she gave him that helped him to “create Charlie as authentically as possible”.

“Something as seemingly simple as you and I just standing up from the couch is Herculean for Charlie to do, and also from the standpoint of the story, it's a major plot point,” says the actor.

“The very way that you utilise his mobility, or lack thereof, is very important.”

Charlie teaches English classes online from his couch, always with his camera off, not wanting to reveal how he looks to his students.

His best – and seemingly only – friend Liz, played by The Menu's Hong Chau, also serves as his nurse and carer, in a complicated role that sees her scold and chastise him while also acquiescing to his food addiction and bringing him supplies of fast food.

Stranger Things' Sadie Sink also stars as Charlie's estranged daughter Ellie who, at 17, is resentful of her father's lack of presence in her life since he left the family when she was a toddler.

Samantha Morton also makes a brief but brilliant appearance as Ellie's mother Mary, the ex-wife who had her heart broken by the protagonist yet still retains a lingering tenderness for him.

Charlie's other regular visitor in this pivotal week is Thomas, a young missionary played by Ty Simpkins who is determined to save his soul, and whose interactions with Charlie leave us with a pertinent message about the power of human interaction.

“Hong Chau, incredible,” praises Fraser.

“You could make a whole movie about Liz – who is she at work, who is she at the grocery store…

“Sadie is prescient. She's a young woman with a talent that just is extraordinary. I had a front row seat to watch her just give a gold star performance every single take of every day. And to me, that was thrilling, to join her in that journey.

“And Ty Simpkins, also really great, I watched him earn his stripes on this movie, and Samantha Morton – let's not forget how wonderful she is.

“She walked in with a built-in history of 30 years' relationship… I genuinely felt sad that my imaginary marriage to Samantha Morton didn't work out! She's so terrific.”

Through The Whale, both Aronofsky and Fraser say that they hope to foster a greater understanding and empathy for people living with obesity – a reconfiguration of beliefs and an eradication of bias.

Having the film be such an intense and comprehensive character study was important to Aronofsky because, he says, “I just think that any character that you don't expect to connect with, and then you do connect with them, and they make you feel in a deep way, ultimately makes us more human”.

“I agree. Don't judge books by their cover,” adds Fraser.

“I can almost guarantee you that by story's end you're going to be reconfiguring maybe what you've thought about certain beliefs you held prior to this story. And I think that's good. I think we should be challenged that way.

“Ending the bias against those who live in bodies the size of Charlie is something we would do well to dispense with.”

The Whale is in UK cinemas from Friday, February 3.