When this film appeared in 1960, Peter Sellers had become an internationally-acclaimed star of comedies, but had never been seen in a serious drama like this violent thriller. People were so unused to see him playing someone unpleasant and aggressive that the film was a great critical and financial flop. Sellers himself, perhaps defensively, dismissed his performance sarcastically as "my attempt to be Rod Steiger". However, over the years, the film gained a small, but vociferous cult following and Sellers's work in it has been much praised.
When Richard Todd is talking to the police outside the run down building that houses Mervyn Johns' flat, there is a peeling poster on the wall advertising the 1955 production of the play "The Remarkable Mr Pennypacker" starring Nigel Patrick and Elizabeth Sellars.
Peter Sellers - cast as the gangster of the film - brought the said character home with him after filming had finished for the day. The character proved a terrifying experience for his wife and children.
In recent years, both the performance of Peter Sellers and the film have gained popularity.
The opening and closing credits feature the song "When Johnny Comes Marching Home". The melody to this song would later be featured prominently in the score to Dr. Strangelove which also stars Peter Sellers.