SLEUTH: this is how to make cinema out of theatre

SLEUTH is that rare breed of film based on a stage play where the obvious theatrical origins don't act against it. Too often the film version of a hit West End or Broadway show looks like little more than a filmed version of the stage production. The biggest giveaways are too much of the action taking place in very limited number of settings, and dialogue that's written to be declaimed rather than spoken.
Anthony Schaffer's SLEUTH has both yet it never feels constrained or 'stagey.'
There's two main reasons for this - Laurence Olivier's wonderfully entertaining performance, and Joseph L. Mankiewicz's direction. 

Olivier's Andrew Wyke is magnificently hammy yet totally believable. An egotistical, arrogant, self-obsessed, upper class, writer of detective thrillers Wyke lives his life as if he's on stage, declaiming, proclaiming, enunciating, and orating rather than simply speaking, and accompanying everything with dramatic gestures, even though he's only playing to an audience of one.
That one is Michael Caine as Milo Tindle, an aspiring celebrity crimper, who's having an affair with Wyke's wife and wants to marry her. The two men come together when Wyke invites Tindle to his country manor to discuss the situation. What starts out as a civilised conversation soon descends into a battle of wits as Wyke bullies his younger rival into playing an increasingly dangerous game designed to resolve their mutual problem.
The action all takes place within the overdecorated confines of Wyke's stately pile yet it never feels claustrophobic. Mankiewicz breaks the story free from it's theatrical constraints by having his camera and his two main characters constantly on the move, creating a liberating sense of continuous forward motion.   
This effect is enhanced with the use of cut away shots of Wyke's collection of mechanical toys, highlighting and emphasising individual details in a way that's simply not possible in a stage play. 
It's almost impossible to write about the film's biggest weakness without giving away a major plot twist so I'll just say that I figured it out the moment it first appeared on screen and if you're giving the film even half your full attention you'll figure it out too.
But even if you do work it out before it's revealed there's still enough in SLEUTH to make it an enjoyable viewing experience - Olivier's performance alone is worth the price of admission.