To the pantheon of Gone Girl–type bad girls you can now add Amber Patterson, the manipulative heroine of The Last Mrs. Parrish, who sets her sights on wealthy couple Daphne and Jackson Parrish.The devilishly ingenious debut thriller from Liv Constantine, the pseudonym of sisters Lynne and Valerie Constantine. Lynne and Valerie wrote the book despite living hundreds of miles apart. Here, they talk about their process.

Lynne: Most sisters argue over borrowed clothes or who had it easier growing up. My sister, Valerie, and I have disagreements about our imaginary friends. Are you nuts? She wouldn’t wear that, say that, do that, think that. We may want to blurt that out, but our discussions are in fact much more diplomatic. Do you really think a millionaire wouldn’t know anything about designer labels? Would a famous and eccentric author drive such a boring car? Does a teenager even know what a disco party is? Is it really necessary to use THAT word? (The last comment usually applies to a sex scene.)

Valerie: It’s funny. Lynne will ask me to cut something that she thinks is too explicit, and I’ll say aren’t you the one who just wrote the scene that made my husband blush? But we have a rule--veto power. If there’s something either of us can’t stomach, it has to go.

Lynne: Yes, that works well for us. Fortunately, Val and I are mostly on the same page so it’s not too often that the gavel comes out and only very occasionally that a heated discussion ensues.

Valerie: But there was that one time when we first began writing together when you wanted our protagonist to be more naïve and chaste than was believable. I think you felt like we would set a bad example or send a wrong message if she succumbed to any kind of temptation. We went round and round on that one. But in the end we agreed that it was vital to ask the question “what would the character do?” and go with that.

Lynne: That’s right. You won that one.

Valerie: It’s not merely a question of winning or losing a point. The great thing is that talking it out together always brings us back to the craft and what it asks.

Lynne: Still great to win, though.

Valerie: Does that sound just like a little sister talking, or what? But people ask us all the time how we can write together, especially when we live so far apart. It’s easy to be in touch today, between texting, email and FaceTime, so we’re able to keep in close contact as the project unfolds.

Lynne: Emails fly back and forth as we send each other images of what we imagine our characters look like, houses that would be suitable for them, clothes they would wear, cars they would drive, and other details that flesh out the story world we’re creating. Most of the collaboration is great fun, and we have lots of laughs developing our characters and our plotlines. We try hard not to offend each other and to make any criticism constructive.

As we wrote The Last Mrs. Parrish, our first thriller together, we discovered what works best for us is to divide and conquer. The novel has two protagonists, both strong female characters, so it made perfect sense for us to each take one. This eliminated the need to merge our voices. Two women. Two perspectives. But we crafted the characters together, both felt ownership of each woman, because it’s vital that we both understood them inside and out. We knew that we’d achieved our goal when we were able to write several chapters in the other’s section and they still rang true.

Valerie: And of course, Daphne and Jackson are central characters in Amber’s point of view, and she in Daphne’s. So, Lynne and I had to know what each woman would say, how she would react, what her mannerisms were in order to keep it authentic.

Lynne: Amber is wicked and Daphne good. Guess whose voice is more fun to write? Being the nice sister, I agreed to give Valerie Amber, and I took Daphne.

Valerie: Hmmm. Not sure Mom would agree with that characterization, but then again, you are the baby and used to getting your way.

Lynne: I think we’re getting a little off course here! All the characters were fun to write and while Val spent a lot of time with the devious Amber, I got to hang out with Jackson Parrish, who took me places I would never go in real life.

Being true to character also meant going “off script” so it was important for Valerie and me to talk daily and apprise each other of any new plot twists. One of the great things about having a writing partner is the immediate feedback you get on a scene. Sometimes I call Valerie before emailing her a scene and tell her I want her to read it while we’re Facetiming. That way I can see her reaction and know right off the bat if I’ve struck the right chord with a given scene or story change.

Valerie: Right. I can always tell by the tone in Lynne’s voice when she’s excited about something new for me to read. Lynne and I FaceTime daily, usually around 4 or 5 in the afternoon, and those appointment times are “written in ink.” In these sessions, we review our work for the day and plot our characters’ next moves. But occasionally I’ll get a call from Lynne in the morning. She’ll open with, A great idea came to me while I was walking the dog. This might sound crazy, but just listen first. And sometimes she gets a similar call from me. Her ideas come to her while she walks, mine while I lie in bed at night thinking about our characters. The process always feels organic, a living, breathing story that goes off in directions we never predicted, much like life does. And when one of us is confounded about where the story is headed, we tell the other to relax and let the character lead the way. It sounds mystical and mysterious, but it’s very intuitive. We live with these characters every day, know their secrets, feel their fears, internalize their longings. And long before they appear on the page, we together create their being––their looks, their past, their beliefs––those things that work together to make them who they are. It doesn’t matter that many of these details will never make it into the story. They are nonetheless vital in creating believable characters.

Lynne: I totally agree. It’s also important to know your strengths and weaknesses. Valerie and I complement each other. It’s not unusual for me to write a scene and type in the middle of it: ADD MORE DESCRIPTION, all in caps. It’s like magic. I send it to Val and it comes back with great details. Meanwhile, she’ll send me a scene she’s written with a note saying RAMP UP THE DIALOGUE IN THIS ARGUMENT, and I’ll turn up the tension. In our case, two heads are better than one.

Valerie: There’s the advantage of being the youngest of four—she’s used to voicing her opinion and is great at writing conflict!

Lynne: I did have three older siblings to learn from and who wore down Mom and Dad by the time I came along.

Valerie: That’s true. But I must admit, Lynne is the more disciplined of the two of us and sometimes is frustrated by my propensity for letting life get in the way of my writing. It has been a slow process for me to let go of other “stuff” and plop myself in front of the computer. I was always the student who wrote her term paper the night before it was due. I have a love/hate relationship with deadlines.

Lynne: I’ve come to believe it’s all in the genes. I tend to get uptight about deadlines, even answering emails. Val’s had to ground me a little, remind me that it’s okay if an hour goes by before we respond to an email from our editor. We’ve learned to meet somewhere in the middle.

Valerie: It’s been said that writing is a lonely profession. In our case, nothing could be further from the truth. It’s brought us closer, given us great belly laughs and great insights into our own psyches. And no one understands the anguish of rejection or the joy of accomplishment like your co-author. The bond of sisterhood is a precious gift. To be lucky enough to create together makes it priceless.