The Wire star portrays West in new U.K. TV movie Appropriate Adult, which centres around the interviews West gave to cops after his arrest in 1994.
The actor reveals one of his siblings was horrified when he told his family of his grisly new role - but he was determined to play the killer.
He says, ""My eldest sister thought it was an appalling thing to do. Many people do think that but I'm certain when people see the drama they will feel differently.
""It's an intelligent and respectful but nevertheless provocative look at an aspect of our society that is very present and, if anything, is getting worse. There are thousands of people in Britain who go missing every year and a proportion of them never turn up again. I realised from looking into Fred West's story that he wasn't a loner who lived in a vacuum.
""I read the scripts and realised that it was not centred on Fred West and I overcame my initial moral qualms. I thought that it was something that could be both an incredible acting challenge and also something of serious intent that was not exploitative. If it was a film just about Fred West and his crimes, it would be impossible to justify.""
Fred West was charged with the murder of 12 young women and girls, but hanged himself in his prison cell in 1995 before he could face trial.