When I first met Scott Payne, with whom I co-wrote Code Name: Pale Horse—How I Went Undercover to Expose America’s Nazis, he was wearing what I would come to learn was his usual uniform, no matter the season. Backwards baseball cap, sleeveless T-shirt to reveal the bulging tattoo-sleeved arms, jeans, and cowboy boots. Looking at this man, who was my opposite in so many ways, I wondered what the hell I’d gotten myself into.

A longtime newspaper foreign correspondent and documentary filmmaker, I had also written nonfiction books before. But I had never been a co-writer. Co-writing, as I would learn, is essentially ghostwriting, but with your name also on the book. So not only would I be writing with Scott, also known as F.B.I. Special Agent Scott Payne, I’d be writing as him. I’d read J. R. Moehringer’s highly entertaining New Yorker piece, “Notes from Prince Harry’s Ghostwriter,” and most of it sounded terrifying.