If you’ve ever wondered what Han Solo did after the Battle of Endor, or whether Doctor Who would enjoy the Isle of Lewis, then you’re in luck. These days, fans of the biggest movie and TV franchises can discover depth and texture in the lives of their favourite characters with a wealth of tie-in fiction.

The novels aren’t just movie adaptations, spin-offs or fan fiction, they’re added to the canon of the film or TV series that inspired them – stories that haven’t made it to the screen but are officially part of those universes.

The latest is Darkness on the Edge of Town, a novel by Adam Christopher set in the world of Netflix’s horror series Stranger Things. Christopher’s novel moves away from the series’ fictional town of Hawkins and goes back to the 1970s, when police chief Jim Hopper was a cop in New York City.

The third series of the TV drama is due in July, so when the US publisher Del Rey approached Christopher with an idea for a story about Hopper hunting a serial killer, he had to make sure the novel was a perfect fit in terms of tone and continuity.

“My editor and I worked closely with the Netflix team – including Paul Dichter, one of the staff writers – to get everything aligned with the TV show,” Christopher says. “As with any licensed work, there are a lot of eyes on it, and there are a lot of channels it has to go through for comment and approval – everything from the way the characters speak to detailed timelines is talked about and confirmed.”

It may be easier work for writers – like Christopher – who are already fans, but he says it’s crucial not to get too close to your material. “A story can easily become too fannish and you can get lost in minutiae that just turn readers off, so you have to know where to draw the line. You have to be true to the property, but you’re also responsible for writing a compelling narrative.”

The American author Chuck Wendig, who has written a trilogy of novels set in the Star Wars universe after Return of the Jedi, also counts himself a fan. That may not be an absolute requirement, but for all the freedom writers have to invent new characters and situations, staying faithful to the source material is the bottom line.

“The movies are king,” Wendig says. “Everything follows behind them in Star Wars.” With the lion’s share of money and time devoted to the cinematic versions, writers have to make sure everything moves together at the same time, “not disagreeing with the films but also not stealing their thunder”.

Jenny Colgan’s Doctor Who novel Dark Horizons featured the 11th Doctor, portrayed by Matt Smith. Photograph: Todd Antony/BBC

Like both Wendig and Christopher, Jenny Colgan made her name with her own original fiction before she was approached to write licensed novels. Her romance bestsellers have have brought her enough attention to fuel a second writing life as JT Colgan, writing adaptations of existing Doctor Who episodes as well as original novels set in that world.

For Colgan the key is to get everything approved in advance. “There’s no point just going for it and having to throw all the work out of the window if the people at the top have already decided the character can’t swim,” she says.

Working within such strict constraints can sometimes be challenging, Colgan continues. “The most common thing to happen is you hit on something they want to use for the TV show, which obviously takes priority. And when you’re dealing with a long-running property you have to watch for duplication – there are, for example, not a lot of British historical figures the Doctor hasn’t met.” But there’s fun to be had bending all these rules. “I always feel like I’ve scored something if I get something through – for example, I got through the way the Doctor smells (a pleasant mix of chalk dust, lime and diesel) and how that stays the same through every regeneration, so I feel that’s my building brick in the universe. And I invented River’s sonic trowel, which then turned up in the telly version.”

‘If I got something wrong, fans would have had no qualms in voicing their concern’ … an audience awaits the premiere of Star Wars: The Force Awakens. Photograph: AP

As the market for tie-in fiction has expanded, so have the ways for fans to voice their enthusiasm or vent their frustration with any new material – enough to give Christopher pause for thought. “With a phenomenon like Stranger Things, I was very much aware that was I was creating would become part of the fabric of the show – officially – and I also knew that the fans would really be paying attention,” he says. “If I got something wrong, they would have had no qualms in voicing their concerns.”

Fans may be protective of the stories they love, but new stories can also stir up strong emotions – as Wendig discovered when the first novel of his Star Wars trilogy was swamped with angry reviews.

“The negative feedback can be pretty overwhelming,” he says. “For me it was complicated by a specific kind of toxicity: fans mad that the book contained ‘politics’ in the form of LGBT characters or people of colour or what-have-you. I don’t feel comfortable pushing back on people who simply didn’t like the book, but I did feel a certain responsibility to push back against some of that specific brand of toxicity.”

Even though they’re working with someone else’s characters, Wendig continues, writers must remember where their duties really lie. “There’s a sense of responsibility writing any story, and that responsibility is first to yourself, I think: telling the best story you can in the way that you choose to tell it. So you can be proud of it – because though it will say Star Wars, it will also say your name, and so you have to be satisfied with what you’ve done first and foremost. And if you’re a fan of that world you have a standard to hold yourself to, which is: does this please me both as the author of the work and as a fan who might be reading it?”