Billy Wilder’s ‘Witness for the Prosecution’, adapted from Agatha Christie’s play, is a wonderfully entertaining courtroom thriller that manages to be both deadly serious and frequently hilarious. It’s a murder mystery about whom we can trust, but also about the dangerous temptation to believe that we’re clever enough to recognize the truth when we see it.

Charles Laughton plays Sir Wilfrid Robarts, a brilliant barrister recovering from a heart attack, whose doctors have instructed him to avoid excitement and restrict himself to the occasional undemanding case. Naturally, he does precisely the opposite.

When Leonard Vole (Tyrone Power) arrives seeking representation after being accused of murdering a wealthy older woman, Sir Wilfrid finds the temptation irresistible. It’s rather like the beginning of a film noir: an apparently innocent man arrives with a story that may or may not be true, and a seasoned professional becomes increasingly drawn into his predicament. Leonard stands to inherit the dead woman’s fortune, which gives him an obvious motive. But Sir Wilfrid is convinced of his innocence.

The trouble is that Leonard’s only apparent alibi comes from Christine (Marlene Dietrich), whose relationship with her husband is considerably more complicated than it first appears. And when questions arise about whether Christine is legally Leonard’s wife at all, the implications for her ability to testify become central to the case.

The film’s title, of course, tells us that somebody will become a witness for the prosecution. But who, precisely, is telling the truth? What makes ‘Witness for the Prosecution’ so enjoyable is the extraordinary balance Wilder achieves between suspense and comedy. Laughton is magnificent as Sir Wilfrid, a man whose formidable intellect is matched only by his determination to ignore medical advice.

His battles with his devoted but exasperated nurse (Elsa Lanchester), provide some of the film’s funniest moments. She’s constantly trying to preserve his health, while he’s doing everything possible to undermine her efforts. There’s even a wonderfully eccentric moment when Sir Wilfrid uses the reflection from his monocle to shine light into somebody’s eyes, apparently hoping to establish whether they’re telling the truth.

It’s an absurdly theatrical technique, but entirely in keeping with a character who regards the courtroom as both a battleground and a stage. And that raises one of the film’s most intriguing questions: just how reliable is Sir Wilfrid’s judgement? He believes he can recognize innocence. He believes he understands human nature. He believes that experience has given him an almost instinctive ability to distinguish truth from deception.

But has it? There’s something distinctly Hitchcockian about the proceedings, particularly the wrong man scenario in which somebody accused of a terrible crime must somehow persuade others of his innocence. Yet Wilder adds a more mischievous dimension. Like Christie’s ‘The Mousetrap’, the film depends upon the audience constantly reassessing what it thinks it knows. Every revelation appears to clarify matters, only for another detail to cast doubt upon our assumptions.

The performances are perfectly pitched. Tyrone Power gives Leonard an appealing vulnerability, while Dietrich is wonderfully enigmatic as Christine, a woman whose motives remain tantalizingly difficult to interpret. The courtroom sequences are gripping, but what really makes the film work is its understanding that a trial is itself a form of theatre. Witnesses perform. Barristers construct narratives. Juries decide which story they find most convincing. And the audience is invited to do exactly the same.

There is, of course, a celebrated twist ending, which I wouldn’t dream of revealing. Suffice it to say that ‘Witness for the Prosecution’ is one of those rare mysteries that makes you feel increasingly confident you’ve worked everything out, only to remind you how easily confidence can become gullibility. The brilliance of the film is that Sir Wilfrid isn’t the only person on trial. So is our own ability to tell when somebody is being mendacious.