When Joanne Harris wrote Chocolat, her novel of morality and magic set in a cloistered French village, she did not expect it to be published, let alone succeed. Her agent thought that it was “very unfashionable writing” and “wasn’t at all the kind of thing he felt would be commercial”. Now, 25 years later, after more than 1m sales and an Oscar-nominated film adaptation starring Johnny Depp, Harris is writing a prequel.

In the original novel, protagonist Vianne sets up a chocolaterie – selling champagne truffles, oyster-shaped pralines, chocolate-coated pains d’épice – at the beginning of Lent, drawing the ire of the town’s priest, who fasts and sees chocolate as sinful indulgence. In the prequel, due to be published in March 2025, readers can expect to learn more about “how Vianne becomes Vianne”, says Harris.

The prequel, titled Vianne, will see the titular character arriving in Marseille from New York, getting a job at a bistro, and eventually becoming involved in an emerging chocolaterie. However, the chocolatier has secrets, one of which “is likely to cause [Vianne’s] life and his to be overturned”, says Harris.

Since Chocolat’s publication, Harris has written more than a dozen novels, which move between magical realism, thriller, fantasy and historical fiction. Her latest, Broken Light, is about Bernie Moon, a woman nearing 50 whose menopause reawakens her ability to look into people’s minds and manipulate their thoughts – a skill that has lain dormant since Moon was a child.

In 2020, Harris was diagnosed with breast cancer. That experience, particularly the “idea that things can happen to your body that you don’t like and that you don’t understand”, became a starting point for writing about Moon’s “sometimes quite frightening” menopausal symptoms, she says.

Harris’s novels often engage with gender politics, and Broken Light is no exception. Early on, a woman is killed in a local park – a storyline partly inspired by the murder of Sarah Everard, says Harris. “It became a very angry story about men and their responses to women.”

Beyond her novels, too, Harris has expressed her views about gender and her support for trans rights online. “I know a number of authors who would be very uncomfortable engaging in public on things that they believe in, and I think they absolutely have the right not to, and I also think that people who feel comfortable doing it or feel compelled to do it or have strong feelings should also be able to do it,” she says. “Some of us are happy to be known only through our books, and some of us are like me” and “don’t see the line between the books and the world”.

Between January 2020 and January 2024, Harris was the management committee chair of the Society of Authors (SoA), and during that time, she saw off a vote to oust her. The move came after Harris posted a Twitter poll in the wake of the stabbing of Salman Rushdie – and after a death threat to JK Rowling who had expressed solidarity with Rushdie – which asked whether authors had ever received a death threat. At the time, Harris said she felt that the criticism she was facing had “nothing to do with the [SoA], and everything to do with my support of the trans community.”

In the vote, 81% of members ultimately backed Harris. Asked what she makes of the vote now, she says that “these things sometimes happen. The SoA is a democracy.”

“It is healthy in any organisation that any voice should feel that they are heard and represented, even if what they are saying is something that the majority don’t agree with, or something that the society doesn’t endorse,” she adds. “We have 12,000 members, and they all have different opinions on different things, and there are bound to be disagreements. It is important for the organisation to still be able to say ‘We represent all of those people’.”

Despite the controversy, Harris remains an active user of X (formerly Twitter). “I always knew that social media was full of the best and worst of everybody, this isn’t something new,” she says of the criticism she received. “Just because something that I said may have made the papers at one point doesn’t mean that I hadn’t experienced similar situations before.”

Social media plays a major role in Broken Light, in which a character is drawn to a conspiracy theory that a drug, MK2, has been created by women to poison men, destroy their masculinity and eventually launch a “gender coup”, taking over all positions of power and “reducing men to slavery”.

Harris thinks conspiracy theories arise because social media feels “very intimate, and yet it really isn’t – we could be talking to anybody.” Interacting with users repeatedly “builds a kind of trust, which is based on this false intimacy”, and leads to the propagation of misinformation.

While the Chocolat prequel will take place offline, lies still spread: Vianne comes to realise that the chocolatier is living a double life. As his “carefully constructed web of lies begins to unravel, Vianne finds herself caught between loyalty to her friend, and the fear that her own secret – a secret her mother has kept all her life – may itself be discovered,” says Harris.

The prequel will see Vianne reinventing herself, looking to get rid of her mother’s influence. “People in my books, as in life, are always the product of their past, and often find it hard to leave behind the choices that have made them who they are.”

• Broken Light is published in paperback on 28 March by Orion. To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply.