The movie improved this in many ways, particularly with the supporting characters: in the play the young man and woman are just nitwits without any real connection to the recently deceased, and there's no auntie character. But the central concept remains and is strong - the two thrill killers, bumping off a young man and inviting his family and friends around for tea and crumpets afterwards.
The character of the professor was more vivid here - a war veteran who has an injury and is sullen and dark; a lot more interesting than James Stewart in the film (although had Cary Grant or James Mason played the part, it could have been totally different). There's also more fighting in the play with the professor fighting away with a cane. Marvellous entertainment.
(I saw this in 2010 at the Chatswood Zenith Theatre and in 2011 at the Bondi Pavilion. The latter production was superior, helped by Josh Quong Tart's brilliant performance as Rupert. Also Iain Sinclair's direction was vigorous, including an opening rape and murder in the nude, to really convey the horror of the crime. In both productions however some actors have a tendency to ham it up with accents and characterisation.)