The Wire star appears as the real-life murderer in upcoming TV drama Appropriate Adult and told The Sunday Times that "very extreme characters are a gift", but admitted that his wife was "revolted" when he was first approached for the role.
He said of Fred West: "Everyone who came into contact with him thought he was a good guy. Salt of the Earth. The police all liked him. He escaped detection for 25 years because he was likeable and pitiable. Rose was not."
The actor added that he identified in some ways with the killer's "attitudes to sex, sexual fantasies, dark stuff" but added that he had to pull himself back from sympathising when "my conscience said stop".
"This is very, very, very dangerous territory, but necessarily one has something in common," he continued. "You have to find common ground and that common ground is the basic part of you."
He later said of Fred West: "He lied all the time. At first he lied to get off, then to get off with a light sentence, then getting Rose off, then blaming everything on Rose.
"Then it was, 'I'll give you one body a year so I get one day out of prison every year. I won't give you all of them, but if I do one a year, I get a day out of prison each year'. I mean, that's crazy."
Dominic West has admitted that he found his research for the project "disturbing", while co-star Emily Watson has promised that the history is "handled very sensitively" in the drama.
Fred West's daughter Anne Marie Davis has criticised ITV for producing the programme.