This project was done in a hurry when a proposal to make a film of Ned Kelly was axed by MGM due to budget worries (at this time Tom Jones had been filmed but not released).
It is not clear why Riesz decided to make a film of this play. Clearly, the part of Danny is tailor made for some show-off acting and Finney grabs the bull by the horns here. His only real mistake is to put on a silly 'boyo Welsh accent. True, the character was Welsh in the play but that's because the part was written by the playwright to play himself. Otherwise, there is no dramatic need for Danny to have a Welsh accent and Robert Montgomery didn't bother in the 1937 version.
The main difference between the film and the play is that the film reveals its hand in the first minute that Danny is a psycho killer. In the play, its not clear until near the end and much of the dialogue are cat n'mouse exchanges between Danny and Olivia.
The result of this is to create a somewhat boring film; you know who the killer is and thus spend an hour and a hour waiting for him to explode. When he does, it has little logic and, to an audience used to the likes of Psycho and its rip-offs like Homicidal, seeing Finney deliver one blow to an off-screen body that you never get to see was always going to be a serious let-down. His retreat into gibberish at the end I thought was uncharacteristic and a cop-out. You never find out who the real Danny is and why he has acted like he has done. I did enjoy the 'hangbags' between Sheila Hancock and Susan Hampshire on the high street of an authentically 60s wet Hertfordshire town.