Joanna Hogg was extremely nervous about showing her mother her latest film, The Eternal Daughter.

The writer-director of The Souvenir films had, again, mined her own life for material and inspiration.

Here she wanted to make something about a woman about her age, in her 60s, and her mother on a trip together.

It would be a ghost story, in a way, with conversations about memory, regret, life and happiness.

But she never got the chance to talk it over with her mother, who died while Hogg was editing the film.

And she is feeling a bit fragile a few hours before its world premiere at the Venice International Film Festival, sitting beside her star and lifelong friend Tilda Swinton, who plays both the mother, Rosalind, and the daughter, Julie.

Hogg is not the only one feeling this way, either. This is the kind of film that gets under your skin.

And no-one is quite sure if they will make it out of the interview with dry eyes. (Spoiler: they do not).

“Maybe we’ll all just have a cry together,” Hogg said.

“I have tissues,” Swinton responded, handing one to Hogg.

“She was so looking forward to seeing this film. She loves ghost stories – loved ghost stories,” Hogg said.

“I was never brave enough to tell her what the film was about. But she probably knew because she was very intuitive.”

It is something Hogg had been mulling for many years.

There was a false start in 2008, but then she was emboldened by Swinton’s portrayal of the mother, Rosalind, in The Souvenir films. They wanted to go deeper into this woman who was a child in England in the Second World War.

In The Eternal Daughter, Julie would take her mother back to the large estate where she lived during the war, now a hotel, and ask her about her memories, with the idea that it would become a film.

The initial conceit was for Swinton to play Julie and to cast another, older actor for Rosalind. But Swinton had another idea – what if she just played both?

“It became a completely different film,” Swinton said.

“It was not about a relationship between two people. It was about something much more profound and mystical and psychiatric and painful. It became much deeper.”

Swinton, whose mother died a decade ago, talked often with Hogg about surviving that loss. Then Hogg suffered the same after the shoot. Though her mother was in her early 90s, it came as a surprise.

“As I was making it, I was wondering how I would be able to show it to her,” Hogg said, wiping away tears. “As a daughter I’m sitting here still feeling very guilty about what I’ve done, that somehow I’ll be struck by lightning, that I’ve done something bad.”

That anxiety that she felt then and is feeling even more deeply now became part of the fabric of the film.

Julie too feels guilty for wanting to make a film about her mother and says that it feels like trespassing.

It is something Swinton can relate to as well. Both are the artist children of non-artists of a bygone generation.

“That in itself is a burden,” Swinton said. “Joanna and I share this sense of deep shame. The idea of being as vulnerable and exposing and having a kind of emotional relationship with the world feels so transgressive and such a betrayal.

“We have felt it all the years of making our work. So we are a mess today.”

They even wondered occasionally if the film would mean anything to anyone but them. It was so personal.

But as with many great films, though the conversations and anxieties presented in The Eternal Daughter are theirs, the specificity also makes it universal.

On set, the small crew would also contribute their own stories to the process. Everyone was personally invested, which Swinton said is rare when you are dealing with such “emotionally expensive” material.

“Having said that, it was so joyful,” Swinton said.

Hogg continued: “Yes, the darker the film, the lighter the shoot. (Ingmar) Bergman was an example of that. He had a great time on shoots.”

Then, of course, there were the logistics of filming long conversations between two characters being played by the same actor.

Hogg and her cinematographer made what Swinton called a “radical filmmaking choice” to not shoot the typical over-the-shoulder angle that establishes an audience in the scene, but to just shoot Julie and Rosalind individually.

Sometimes Swinton would spend full days shooting as Rosalind and the next as Julie, and other times it was half days with a switch in the middle.

“The skill behind that was quite remarkable because she is shapeshifting and each day swapping from one character to another and doing it with no gimmicks,” Hogg said.

“It was strangely symbiotic and quite easy,” Swinton added.

As with all of Hogg’s films, there was no traditional script. The conversations are improvised, which allowed Swinton and Hogg, the one conversing with Swinton off camera as Julie or Rosalind, to follow their noses.

“Joanna’s way of working, and the way of working that I am now absolutely devoted to, is the most, for me, inspiring and responsive way of working I can imagine,” Swinton said. “I’m loathe to work any other way now.”

“I find it extremely inspiring to ask oneself what would one say next,” Swinton continued. “It’s a revelation.”

The editing of it, however, was “quite complicated,” weaving together thousands of hours of improvised material.

With every film, once it is done, Hogg says goodbye and lets it out into the world.

The Eternal Daughter, which is playing in competition at Venice, will screen at several more festivals before A24 sets a release date.

“It’s hopefully a gift for people. We’ve really opened ourselves up,” Hogg said. “And our parents would be horrified.”

Swinton added quickly, “Or not? Maybe not.”

Then, as if rehearsed, the two lifelong friends said in unison: “Maybe they wouldn’t be.”