Dominic West has described how he was attracted to playing the role of serial killer Fred West and often had to stop himself from sympathising too much.
The Wire star, 41, said he identified with aspects of the killer's character, "attitudes to sex, sexual fantasies, dark stuff..."
But he continued: "This is very, very, dangerous territory, but necessarily one has something in common. You have to find common ground and that common ground is the basic part of you."
He told the Sunday Times Magazine he often had to pull back from sympathising too much when "my conscience said stop".
West, an old Etonian, said he thought twice about accepting the controversial role.
But he had long noticed a similarity in his swarthy appearance, which was enhanced by a curly wig and sideburns.
And he was able to get into character because, although the killer was a "monster", people who came into contact with him found him "likeable and pitiable".
The actor also said he found much of West's behaviour "crazy", such as his promise to give police a body a year for a day out of prison a year.
The ITV drama Appropriate Adult, which is expected to be aired later this year, focuses on Janet Leach, the volunteer who sat in on West's police interviews.