I have to be careful describing the film Nightbitch. Not because of spoilers, but because there is a very real danger I will walk through it frame by frame. And not because it’s flawless in its depiction of motherhood in the early years – what it does to the self, to relationships, to the body, to one’s orientation to the world. Rather, because I have never seen it told on screen, from the mother’s point of view, with anything like this accuracy.

“Becoming a mother is such an overidealised moment in culture,” says its director, Marielle Heller, words tumbling over those of her star, Amy Adams, when I meet them in London. “And then when you go through it, you’re like: ‘What?! This is not what I expected.’ And then you have a sense of failure, because you assume that everybody else is having the idealised version and that there’s something off with you.”

Adams, who is more softly spoken and conciliatory, chimes in: “I think every mother feels this way: why am I doing this so badly?” Heller continues: “Why does everybody else seem to be doing it so well?” Adams concludes: “I think that contributes to the isolation, this feeling of not wanting to admit your struggles.”

Heller, 45, is recognisable from The Queen’s Gambit, in which she played the heroine’s adoptive mother and manager with a raw intelligence shining out of her eyes. She is better known as a director (The Diary of a Teenage Girl, Can You Forgive Me?, A Beautiful Day in the Neighbourhood).

Adams, 50, looks Hollywood perfect, as you would expect, unless you had seen Nightbitch, in which her transformation into a harried mother such as you might see at soft play is eerily convincing. I ask whether it was hard – after being exquisite on screen ever since her 1999 debut, Drop Dead Gorgeous, in which she embodied perfection (although it was in 2005, with Junebug, that the critics fell in love with her) – to swerve into frumpiness. She seems a bit surprised by the question. “I really wanted to be present in the physical truth of the character. Even though she’s being judgmental of herself at times, I didn’t really judge her. It’s more in the watching that I recognise the judgment I might have.”

Nightbitch is an adaptation of the magic-realist novel by Rachel Yoder. Adams’s character, Mother, has given up her work as an artist to look after her baby full-time; he is about two and a half. Her husband, who continues to work, is a lovely guy – or rather you can catch the scent of the lovely guy he used to be. Now, he is the monster who thinks looking after a toddler is just like loafing about, with company, and doesn’t understand why his wife can’t just be happier, or at least buy milk.

Her maternal love commingled with feelings of unbearable monotony; her sense of having been exiled from the worlds of adult conversation, physical attractiveness, intellectual stimulation, creative spark and cast into a domestic prison where shapes mean more than words: it all gets way too much. Magic intervenes and she turns into a dog. If you love your children, but have ever been driven up the wall by your role in their creation, and you also love dogs, it might feel as though Heller has filched the keys to the vault of your psyche.

In truth, it’s obvious why this story doesn’t get told more often: to complain about the maternal experience sounds a lot like you are not thankful enough for your precious offspring. Apart from anything else, you wouldn’t want that for them – to have the one mother who wasn’t grateful. Heller smiles wryly. “Grateful is the thing we always want women to be,” she says. “It’s very rare to actually feel it when you’re in the middle of something challenging. Then there’s the cycle of: I should be happier, I should be this. And you don’t feel that.”

We also don’t talk about it because, as Heller says: “You’re so sleep-deprived, it’s a blur, those months right after giving birth. I remember thinking: if anyone knew how stupid I feel, how much I don’t feel competent to even operate a car … It would be so bad for feminism if people knew how much I feel like my brain is not functioning.”

Alongside the emotional experience – its nuance and complication, the fact that it’s no rose garden when you were promised one of those – the physical reality of making a person and bringing it forth, the ravages, are so unsparingly depicted in the film that, when I saw it, the audience divided along generational and gender lines. Every woman my age (51) was laughing out loud. Most of those who were younger or male were open-mouthed with disgust. “We’ve been making the joke that this is a horror movie for men and a comedy for women,” Heller says. “There is definitely a sense of: you can’t say this out loud.”

Mother doesn’t turn into a dog entirely by tactful CGI. It starts with hair in all the wrong places. “That’s my own hair,” Adams says. “I grew it out for the film. I was like: ‘Mari, you know, I can really grow whiskers. I can do this for you.’” The scene where she grows a tail is breathtaking: imagine how gruesomely compelling it is to squeeze a boil, but now imagine it huge, and then a thin, scraggy shaft of tail emerges.

“There’s this look that Amy gives at the end of that scene that’s kind of like: ‘Well, isn’t this interesting?’” says Heller, affectionately. “It makes me giggle every time, because it’s so not how you expect her to react. And we talked about that. All transformations have a bit of euphoria to them.”

Adams adds: “I wanted to show this radical acceptance of change. Here we are. What next? Maybe I’m just hairy now. Maybe I have a tail. I feel like that about ageing: oh, so that’s where we’re at this morning. That’s what we’re working with.”

There is no reason you would have seen a woman grow a tail in a film before. What is strange is how, as Heller’s sister told her: “I can’t believe how good it feels to see menstrual blood that looks real in a movie.”

“And I was like: ‘Oh, this is my second movie with period blood,’” says Heller. “I never realised. Clearly, it’s something I want us to show, because I want it to be normalised. I think I’ve always felt that way: from the time I was going through puberty and getting my first period, my body is so weird and gross and interesting, and you never see it really reflected back in those terms. We’re supposed to be perfect and we don’t poop and we don’t smell. I always wanted to see it grosser and realer.”

Next, we talk about incontinence. Even though it isn’t news to me that women have this conversation after having children, I am tickled to be having it with Adams, whose daughter was born in 2010. (I remember going trampolining and a friend saying: “I hope you’ve got your tenners,” and me saying: “It’s really not that expensive,” then realising midway through that she meant Tenas, the incontinence pads.) “I would go to exercise classes and they’d say: ‘OK, now, jumping jacks,’ and I’d think: you’re hilarious if you think that’s happening,” she says. This, for me, is the feminist frontier that menstruation was for Heller’s sister: even as we talked for ages about peeing our pants, I was thinking: of course, I’m never going to write that.

When they were shooting the movie, Heller’s daughter was exactly the same age as the child in it. When she was adapting the novel for the screen, she was pregnant and then had a newborn, which made it easier, she says – details would insistently suggest themselves. And working with small children isn’t the recipe for disaster the patriarchy makes out. “It’s a cliche of acting school that you prepare, you prepare, you prepare, and then you throw it away and you’re spontaneous and present in the moment,” says Heller. “And you can say that’s the goal, to be open to the magic that might happen in the moment. But, man, you stick a three-year-old in there – you can’t be anything but in the moment, and present, and spontaneous. Because they’re never going to do exactly what was planned.”

For Adams, recreating that mother-toddler dyad was almost like slipping into mime. “I’ve always approached my characters with physicality, but working with that little one, you had to be so physically present. It just changes the whole lumbering nature of your walk.”

Midway through the film – and I hope this isn’t a spoiler but rather exactly what you would expect to happen when one character has become so alive with frustration that she is turned into a dog – the parents have such a catastrophic row that the relationship nearly falls apart. I put it to Heller and Adams that this segment feels idealised. “What did you think was idealised – a man saying ‘I was wrong’?” Heller asks. Well, yes, pretty much. I have never seen such a total rollover in the wild.

“This is becoming a bit of a crusade for me, to show men apologising in a movie. In every movie that I put it in, people say that it’s idealised. Or people say we could cut it out, that the movie doesn’t need it, that it’s implicit. And people go: ‘Well, that’s not realistic.’ There’s something really problematic about the fact that we think it’s not realistic for a man to take accountability and apologise.”

The funny thing is, amid all the excretions and the hair and the canine transformation, amid the ugliness of marital strife and domestic drudgery and the kaleidoscope of love, fear and shame, easily the most embarrassing bit of the movie is when Mother goes into the city to have dinner with her friends from the before-times and says something inane, in the “cute thing my toddler said” space. It’s absolutely fingernails-down-a‑chalkboard cringe.

Adams took that bit in her stride: “I embarrass myself all the time, so I have a high threshold of embarrassment. I’m constantly saying something totally ridiculous.” Heller remembers meetings in Hollywood – to “suddenly be in a room full of men and think: ‘I only know how to talk to two-year-olds.’”

This is the final explanation of why this story is so rarely told: having a baby is such a profound, mature and social experience – losing your self and your place in the world, forging a new self and finding a new place – but its language is baby talk and its materials are poster paints, if you are lucky (before that, nappies). It seems it was just waiting for two whip-smart and quite different imaginations – and a bit of magic realism.

• Nightbitch is released in the UK on 6 December