Simpson reveals she flirted with Knoxville and would send him e-mails as Nick slept next to her.
In her new memoir, "Open Book," Simpson opened up about her relationship with Knoxville while also discussing the end of her marriage to Nick Lachey. Around the time she filmed the comedy, she and Nick were already having serious issues -- and started a flirtation with Knoxville on set.
"Sigh, Johnny. The boy from Tennessee, as I coded him in my diary," she wrote in the book. "First off, we were both married, so this wasn't going to get physical. But to me, an emotional affair was worse than a physical one."
"It's funny, I know, because I had placed such an emphasis on sex by not having it before marriage," Simpson continued. "After I actually had sex, I understood that the emotional part was what mattered. And Johnny and I had that, which seemed far more of a betrayal to my marriage than sex."
According to Simpson, they could talk to each other for hours and she loved his "mix of adrenaline and intelligence."
"I could share the deepest authentic thoughts with him, and he didn't roll his eyes at me," she added. "He actually liked that I was smart and embraced my vulnerabilities. He didn't make fun of me, he laughed with me. He believed in me and made me feel like I could do anything."
She noted that Lachey, at the time, did not make her feel the same way. "We were open about the challenges of our marriages and why we felt we had to honor those vows by sticking with the marriage," Simpson added.
While Simpson's BFF CaCee Cobb told her what she was doing was disrespectful to Nick, Simpson and Knoxville continued their flirtation. Jessica said Nick was "suspicious" about what she was doing in Baton Rouge, where they were filming, and eventually joined her down there.
After filming was done, the two costars kept in touch via e-mail. "We wrote these flowery love letters back and forth, often at night with Nick passed out in the bed next to me," she revealed. "We talked about music, and I would listen to the Johnny Cash songs he suggested just to feel like we were still together."
Saying they were like "prison pen pals" kept apart by "our respective spouses," Jessica would delete the messages out of fear her husband would see them. She eventually deleted Knoxville's number from her phone, but they still kept in contact via e-mail.
"I began to realize contact with him was an addiction, and I would make these grand pronouncements that each time was the last time," she wrote. "I was grateful that he had expanded my worldview and made me appreciate so many things about culture, but I needed to walk away. This wasn't a game."
After she and Nick split in 2005, Simpson claimed Knoxville texted her asking how she walked away from her marriage. "You just have to do it," she told him. While Knoxville and wife Melanie Lynn Clapp split in 2018, Simpson said "it was too late for us."
TooFab has reached out to Johnny Knoxville for comment.
"Open Book" is out now.