When you write a novel that becomes a hit television series, you end up sharing custody of your characters.

The people you created from nothing pass through the hands of screenwriters, production teams and actors before they reach the millions of viewers who will then never picture them any other way.

For Liane Moriarty, the novel was Big Little Lies, the show was HBO’s Emmy-sweeping adaptation, and the actors were Nicole Kidman, Reese Witherspoon, Laura Dern, Shailene Woodley, Zoe Kravitz and Meryl Streep.

A decade on, when Moriarty sat down to write the sequel, she had to work out how much of them she could take back.

So nowhere in Big Little Truths does Moriarty spell out the colour Madeline’s hair. She stopped herself mid-line when writing how tall Celeste is. Renata, a side character in the first book, is now fully part of the gang on the strength of Dern’s performance. In one conversation about the book, her editor called Bonnie “Zoe”.

Moriarty’s word for it is “morphed” — her characters and their screen versions folded into each other somewhere along the way, until she could no longer pull them apart.

But if it’s hard to know where Celeste ends and Kidman begins, the sequel stays true-blue to one point. It’s still picturesque Pirriwee, a fictional coastal suburb of Sydney’s northern beaches, not the show’s Monterey.

“I completely saw it all happening in Australia, and their language feels Australian,” Moriarty says. “But maybe some of the American readers, when they read it, maybe they’ll be seeing it in California and thinking these people are just speaking slightly funny . . . because the tone I think is still very Australian.”

The result is like catching up with old friends, that is, if your friendship group had witnessed something that changed their lives and bound them together forever.

Set 10 years on, the Monterey five have mostly moved on with their lives. Jane is married to Tom, Madeline is planning her big 5-0, Celeste is navigating life with spirited twins and headstrong mother-in-law Mary Louise. Yet the ripples caused by Perry’s death are evident.

There’s a new mystery to solve, too, which starts with a severed finger being sent to their kids’ school and, in classic Moriarty form, lands somewhere you won’t see coming until the final pages.

Their kids are all grown up and bring their own stories and dynamics to the gripping world Moriarty has built. Writing a brood of moody teenagers was a challenge she felt ready for, having raised her own.

“I thought I needed to experience it myself before I could write about it, which I have now — I’ve got an 18-year and a 16-year-old,” she says.

“And they did provide me with a wealth of material, although I need to make clear that it’s not about them. A wealth of inspiration might be a better word.”

They also gave her a tip on teen dialogue: don’t attempt the slang. She jokes there will be no six-seven.

“Nobody forgets their teenage self,” she muses. “Even Mary Louise thinks about the schoolgirls being mean to her all those years later, and my mother can access her memories of being a teenager really easily.

“I think it’s just a huge part of your life, and bringing up a teenager is like going through it all again, seeing it all. There’s also the fact that women in their 50s are often going through menopause, which is another big hormonal shift, so they feel like teenagers, too.”

The title change from lies to truth was as strategic a move as it was serendipitous.

“I guess nobody’s actually (truthful) in the first book,” she says. “There are deceptions, and in the second book there are revelations. Although you could also possibly argue that both titles could have applied to the other book, and it was just a neat way of having a title that made sense.

“In fact, originally I was calling it bigger lies . . . then an editor said, ‘There’s too many books with lies now, let’s get the word out’. So the truth is that there was a little bit of circling, but I think that it makes sense. I’m happy with it. It was serendipity that it sort of worked in the end.”

A third season may be coming, though how Kidman squeezes it in among her busy schedule, or Kravitz works around wedding planning to Harry Styles is anyone’s guess.

But none of that is Moriarty’s problem; she’s done her part. The rest she will hand over again — to the screenwriters, actors and audience — and wait to see who they send back.

Big Little Truths by Liane Moriarty, published by Macmillan Australia, $34.99, is out August 25.