The harrowing true crime series focuses on the killer’s coercive and violent relationship with his girlfriend Delia Balmer, portrayed by Line of Duty actress Anna Maxwell Martin, 47.
Shaun, 44, best known as the young Inspector Morse in Endeavour, said he used the technique to try to get into the mind of Sweeney, who he was initially repelled by.
He said: “I watched the documentary about the case and I listened a lot to his voice and looked at his drawings.
“I had them in my room before we went to set every day.”
One of Sweeney’s drawings was a depiction of the headless body of his second victim, Melissa Halstead, 33, who was found in an Amsterdam canal also with limbs missing.
Shaun said: “Ultimately I’m an actor, you know, so you have to come in and do your work. It was an interesting one. It was unusual in many respects but gratifying in some.”
Initially unsure about taking on the role, he said: “I had trepidation. I’m not going to lie.”
Sweeney was given a whole-life sentence in 2011 for two murders – that of Ms Halstead and also of Paula Fields, 31.
● Until I Kill You will air on ITV1 and ITVX on four consecutive nights next month
