As a writer and reader, Gardner prefers fallible characters rather than the "superwomen" who appear in some novels. "I think you can relate to them," she says. Even so, she struggled a little with translating Libby Denbe's voice to the page in Touch & Go.
"There are flawed characters, and then ... well, at a certain point, I think you wonder whether this woman is going through a nervous breakdown," says Gardner. "She's crossed a few more lines of likeability than most of your traditional characters.
"But I think she's so raw and compelling, how fragile she is, that her whole life has fallen apart. There are layers to that ... she's not coping well, she's not terribly well-adjusted, and she's not the typical heroine finding her inner strength."
Gardner says that in the end, she "just had to let [Libby] be for a while, and let her tell her own story"; to take an authorial step back, and not impose her own values on to the voice in her head.
"I think there's an honesty to Libby that readers relate to," says Gardner. "They might not like everything or approve of everything she does but her pain, you can get that. I mean, most of us know people who've gone through nasty divorces or have hard things going on, and I think she represents that person who's got to the point in their life where they're supposed to have it all, and realise that, in fact, they may have nothing at all."
The raw and unguarded Libby is contrasted with another complex "heroine" in Touch & Go: the emotionally well-armoured Tessa Leoni, first seen as a State trooper suspected of her husband's murder in 2011's Love You More.
Gardner, who started her writing career by penning the first of 13 romantic suspense novels as a 17-year-old (selling it at 20), says she brought Tessa back because she was still curious about the character, who'd grown on her. "She'd gone through so much to get her daughter back, at an extreme price to herself," in Love You More.
"One of the nice things about fiction is you can get more closure. How is Tessa getting on now that she can't have the job that she really loved, being a State trooper? Are she and her daughter doing okay, now they're on their own? You know, her husband is still kinda dead. That doesn't magically change."
Tessa was never intended to be a recurring character. In fact, Gardner says she has "never actually intentionally written a series yet". Even D.D. Warren, who has featured in six of Gardner's crime thrillers, the novella The 7th Month, made a cameo appearance in Touch & Go, and was recently played by Carla Gugino in a television movie adaptation, was originally intended as a walk-on part when she first appeared in 2005's Alone. "I needed one chapter with a female Boston cop," says Gardner. "And the next thing I know, everyone is, like, 'we love that character'."
After completing Alone, Gardner realised she was still curious about D.D, and that she still had "more stories" to explore.
The quality of her characters has been Gardner's calling card since her early days as a rookie romantic suspense novelist, writing under the name Alicia Scott. "I think if readers pick up one of my books it's because they really want to read strong characters."
Her evolution from romance to crime happened, she says, because she wanted to take her people to explore bigger, more complex plots: - "bigger ideas, bigger twists, a little more shock value".
She had to grow more comfortable conducting in-depth research, talking to police and others, to write crime.
"One of the things I love about crime, and I do think it's the same for readers, is law enforcement, and the psychology of it," she says.
The two most popular genres in the world share a lot of traits. Gardner herself reads and enjoys both, admitting she's always been attracted "to the idea of dark and stormy nights, 'drop a dead body in', and see what ensues from there".
Romance novels, like thrillers, are page-turners, she says. "Their tension derives from 'will they end up together?'. And many readers who love romance are looking for that page-turning experience, and suspense delivers that as well, although it's more action-based."
I can't help but ask Gardner whether romance and crime are so popular for escapism, or because they're the things readers care about most? "Love and death," she laughs.
Touch & Go (Headline $36.99) is out now.