“Outlander” stars Sam Heughan and Caitriona Balfe have undeniable chemistry in their roles as the series’ romantic leads Jamie and Claire, giving viewers six seasons of passion and romance — even when they were separated by centuries of time.

In an exclusive excerpt from Heughan’s new memoir “Waypoints: My Scottish Journey,” the actor details his powerful first encounter with Balfe at a casting panel.

Ahead of the show starting production, Heughan explains that “the chemistry had to be just right” between him and whoever took on the role of Jamie’s time-traveling lover. The intense read-through that followed between the two actors easily solidified them as the perfect duo to portray Jamie and Claire.

Heughan recounts how he and Balfe were asked by a casting director to perform what would become a famous scene from the series’ first season — a tense exchange between Claire and Jamie that occurs in an episode titled “The Reckoning.” Fans will easily remember Jamie’s tortured line: “You’re tearing my guts out, Claire.”

“Despite their enduring love, the two characters can be quite antagonistic in their relationship. In the heat of that moment, born from love and frustration, we literally began tearing at each other until the director called time on us. It was intense, physically and emotionally. It left us breathless. She was wrapped in my arms and I think we both know that Jamie and Claire had just come into existence together,” Heughan writes.

Heughan also mentions how he became protective of Balfe as “Outlander” began to shoot, noting this was a breakout role for her as an actor.

“It was a big leap for her. I felt that I needed to be protective towards her, as supportive as Jamie is with Claire. In some ways, it seems as if that dynamic developed between us as actors and then went on to amplify and be explored in front of the cameras,” Heughan says.

The actor also shares in his memoir his close relationship with “Outlander” author Diana Gabaldon, teasing that he’s been informed how the story of Jamie and Claire ends.

“Gabs is a wonderful and fascinating woman who has always revealed so much to me about the character’s subtleties and motivations. When the first season went into production she’d written eight novels in a planned series of ten. With the narrative mapped out in her head, she even told me what would happen in the closing pages of the final book. It’s been our secret ever since with only perhaps Meryl Davis, the show’s executive producer, also knowing.”

Listen to the full audio clip from Heughan’s “Waypoints” below:

Set over the course of a solo six-day, 100-mile journey deep into the Scottish Highlands, “Waypoints” offers an intimate journey of self-discovery for the actor. Heughan uncovers personal details about his life, from his fraught relationship with the father who abandoned his family as a boy to drama school rejections, disastrous “big break” auditions, and disordered eating patterns triggered by a demand to meet unhealthy expectations surrounding a leading actor’s physique.

The memoir also features dozens of never before seen photos from Heughan’s life, from his early adolescence to hiking through the highlands to “Outlander” behind-the-scenes shots.