Sarah Pekkanen won’t say much about “The Wife Between Us,” the psychological thriller she wrote with Greer Hendricks. “You think you’re reading about a discarded ex-wife who’s obsessed with her replacement — the younger, beautiful woman her husband is set to marry,” Pekkanen says. “But you should suspend your assumptions.” Pekkanen and Hendricks, who’ll stop at Politics and Prose on Wednesday, met in 2008, when D.C.-based Pekkanen sold her first book to Hendricks, then an editor at Simon & Schuster in New York. Their collaboration is poised to become one of 2018’s biggest hits: It was optioned for film by Steven Spielberg’s Amblin Partners when it was only partially complete. Express talked to Pekkanen about “The Wife Between Us,” and the buzz around it.

Politics and Prose, 5015 Connecticut Ave. NW; Wed, 7 p.m., free.

How did you and Hendricks become co-authors?
Greer and I became very close friends during the course of working on seven books together. We would have long dinners when I went to New York and sit and talk for hours, and when she left publishing, she told me a secret, which was that she wanted to try to write fiction. I had just finished a book, and she was struggling to figure out what she wanted to write.

So I suggested [a joint project] to her, and she said, “Let me think about it for 24 hours.” And she emailed me at 5 a.m. the next morning, about 12 hours later, and said, “I’ve been up all night, I’m so excited, we have to do this.”

You didn’t alternate chapters — you wrote each page together. How?
We talked through every single line. We had Google Docs up so we could see each other in real time as we typed, and we could also have a conversation. Generally, one of us takes the keyboard, we’re both talking, and the other dives in and starts typing simultaneously. We now speak in shorthand — we just say, “This, oh, yes, no, that, OK, got it.” We don’t even need to speak in full sentences because we’re so attuned.

Once a month, one of us will go to the other’s city and get a hotel room, and the second person will come. We wear exercise clothes because when we’re creatively blocked, we walk around the block. We just write like crazy, we reward ourselves with dinner and then the next morning, we go at it again until we have to go home.

Why write a psychological thriller?
Greer and I both took a ton of psychology courses; we both subscribe to Psychology Today. It’s something we’ve both been drawn to — human nature, why people act the way they do.

What was it like to get a movie deal before the book was even complete?
It was crazy. We spent a day talking to different producers who were interested in the material, and we had only written the first act and a synopsis. What really stunned us was that these Hollywood producers asked such tough questions — they were so sharp: “What about this, and have you thought about this?” They really had such different visions for what they might want to do with the material.

But when we spoke to Holly Bario [at Amblin Partners], she was so committed to the project. And we loved the idea that we’re women telling a women’s story, and she’s a strong female exec who has a great record of movies. They did “The Girl on the Train,” and it just seemed like a perfect fit.