Jamie Dornan, when he’s not playing BDSM fanatic Christian Grey in Fifty Shades, plays a serial killer on The Fall – which required some borderline illegal preparation.

Jamie Dornan’s ‘The Fall’ Prep

In order to get into the mind of Paul Spector ahead of the show’s first season, Dornan decided to see what it would be like to stalk a woman.

“The first series, I did do a couple of things to try to get inside [his mind]. On the Tube, which is our underground system,” the actor told the Los Angeles Times, before continuing warily, “Can we get arrested for this? Hold on … this is a really bad reveal: I, like, followed a woman off the train one day to see what it felt like to pursue someone like that.”

“It felt kind of exciting, in a really sort of dirty way,” Dornan went on. “I’m sort of not proud of myself. But I do honestly think I learned something from it, because I’ve obviously never done any of that. It was intriguing and interesting to enter that process of ‘What are you following her for?’ and ‘What are you trying to find out?'”

Dornan did note that he kept his distance while getting a feel for stalking, following the woman for a couple of blocks after she got off the subway. The questionable exercise, according to the actor, did apparently give him a better understanding of Spector.

The Fall, which previously aired on BBC 2 in the UK, is available on Netflix for American audiences for its second season. The series will be returning for a third season, with Dornan and his costar Gillian Anderson.