When the social historian Emily Cockayne first came across an old newspaper article about some “indescribably filthy” letters that were sent to residents of Littlehampton in the 1920s, she knew immediately that she wanted to write about it.

But she never dreamed that recounting the extraordinary real life events she had uncovered in a scholarly book about the history of neighbours would one day net her a film deal for a movie starring Olivia Colman, Timothy Spall and Jessie Buckley.

Wicked Little Letters, a comic thriller about the Littlehampton “poison pen” letters and the libel trials that ensued, will be in cinemas from Friday. It is based on what Cockayne describes as “the weird stuff that develops” in “a neighbourhood that’s gone awry” – a story that she first pieced together writing her 2012 book, Cheek by Jowl: A History of Neighbours. “I was researching neighbours and trying to put together various ways neighbours could be in conflict,” said Cockayne, who is an associate professor in early modern history at the University of East Anglia and worked as an expert consultant on the film.

“I’d found litigation cases and police court cases where neighbours take really minor, petty issues to court like ‘my neighbour stole my clothes off the line’. And then, around 2008, as I was trawling newspapers from the 1910s and 1920s, I hit upon this case in Littlehampton.”

The more she delved into it, the more fascinated she became. “It gave me a lens into obsessive, focused, uncomfortable neighbouring,” she said. “I thought: I’ve got to work out how this neighbourhood actually works, and how this can happen there.”

Colman plays Edith Swan, a “respectable” working-class woman who, in 1919, starts accusing her rough, foul-mouthed nextdoor neighbour Rose Gooding of sending her – and other residents of the small English seaside town – “wicked little letters”. Gooding, who is played by Buckley, was then put on trial for libel. But is Swan the innocent victim she purports to be?

The letters, in which Swan is described as a “foxy ass piss country whore”, inspire some of the funniest moments of the film – but were decried by contemporary newspaper reports as too offensive to print and, in one case, too obscene for a jury to hear.

“To the old bastards,” reads one, addressed to a mutual neighbour of Gooding and Swan. “You dirty cows. You bloody fucking sods … You are bloody dirty or you would clean the yard sometimes, you bloody rotten buggers.”

Another explodes in a rant about a neighbour’s stinking drains: “You bloody fucking flaming piss country whores, go and fuck your cunt.”

“It’s the way they’re living,” said Cockayne, who expanded on her earlier research and wrote in more depth about the Littlehampton libels in her 2023 book, Penning Poison: A History of Anonymous Letters. “Between 1919 and 1944, neighbours started to stop being so reciprocal and caring about each other’s idiosyncratic differences and became more privatised and more judgmental.”

The Littlehampton letters stand out because most missives with obscene language were written by men at the time. “The unusual part of this is that these letters were tapping into neighbouring issues to do with dirty back gardens. It’s a way of understanding neighbouring – particularly feminine neighbouring – at the time.”

The youngest of 13 children, Swan had grown up in a claustrophobic two-bedroom house. At the end of the first world war, she is a 30-year-old spinster who is still living in her childhood home, helping to look after two of her adult brothers – who Cockayne’s research suggests were “very odd” and possibly mentally unstable – and sleeping in a bedroom with her elderly parents. Swan’s father, who is played by Spall, was described by police as “an irritable and excitable old man who would not be very difficult to upset”. Cockayne thinks Swan had “no space to think” and felt trapped and frustrated by her inability to express herself: “She is stuck at home, parenting two weird brothers and increasingly elderly parents.”

That’s when Gooding, 29, moves into the house next door with her husband, her sister and five children (four of whom were born illegitimate and were rumoured to have been fathered by different men).

The two families are then forced to live uncomfortably close to each other. “Rose’s house was basically in the back yard of the Swans’ house,” said Cockayne. “They shared a store, and previously had shared a toilet.”

Some of the letters are signed “RG”, which is partly what convinced the local police that Gooding wrote them. “They think someone like Rose Gooding would be so stupid that she would sign her initials,” said Cockayne.

Although both families were equally working class, the illegitimacy of so many of the children made the Goodings appear less respectable than the Swans. In the eyes of the police, “she looks the part and acts the part”. “To them, she’s an obvious suspect.”

Gooding’s apparent irreverence for propriety might have made living in close proximity to her both abhorrent and attractive. “I think Edith became a little bit obsessed with this family in the garden,” said Cockayne. “I don’t think she wants to be who she is any more. She looks at Rose and she thinks she’s wild and exciting.”

Her favourite scene from the film occurs right at the start, when Gooding returns a washtub her family shares with the Swans by leaving it outside their front door, dirty and full of hair. “That, for me, sums it all up – what these neighbourhoods were like and what’s behind these letters: complicated, entwined lives, shared spaces and the annoyances that can come from that. If people are forced to live their lives like that, they’re going to fall out.”

• Penning Poison: A History of Anonymous Letters by Emily Cockayne is published by Oxford University Press (£20). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply.