Not often, though. The plot here concerns a group of witches in a small village (they live in a converted pub, the Pale Horse) who act as both cover and advertisement for an assassination bureau. It's less a whodunnit than a howdidtheydoit (are the witches really killing by supernatural means?), although there is the ritual unmasking of the master villain at the end. What's striking is that while the story is set, nominally, in the early Sixties (and Enyd Williams' production established a vague sense of period with Dave Brubeck's 'Take Five' as a theme tune), there's absolutely nothing to indicate that it isn't the 1930s. One hint of social change intrudes: at one point Ginger, the Bohemian heroine, refuses to pose as the hero's fiancee, on the grounds that she is obviously the kind of woman who'd live in sin with him; but she spoils it all by agreeing to marry him in the final scene.
There was arguably a smidgin of poetry in the decision to broadcast this for the 50th anniversary of Saturday Night Theatre (Radio 4) - using a pale horse to celebrate a war-horse. But it's clear that the main reason was that murder is a guaranteed crowd-pleaser; and Christie's brand - reassuring, sanitised, rarely bloody or violent, hardly ever involving sex or grand passions - is especially palatable. It has a timelessness about it - a 'classic' feel, perhaps.
Not, however, a 'Classic Serial' feel. On Feedback (Radio 4, Friday), John Tydeman, BBC Radio head of drama, was defending the use of the Classic Serial slot for apparent non- classics (such as Walter de la Mare's novel Memoirs of a Midget) by reference to a 'Classic Serial feel'. A serial that isn't a classic can still earn the label, providing it has the feel.
You could grasp something of what the distinction means from the new Classic Serial, Emile Zola's La Bete Humaine (Radio 4, Sunday), now serialised in three parts, and elegantly directed by Nigel Baldwin. Like The Pale Horse, this is a murder story - to rub that in, the eerie music is by Barrington Pheloung, who wrote the music for Inspector Morse. Otherwise, the resemblances are skimpy. We know from the start who's committed the murder; the motives behind it are entangled in jealousy and bad sex; and the milieu is primarily working-class. Marriage is a fitting punchline to a Christie mystery - the murderer is caught, we arrive at a state of peace, and marriage crowns it all. Here the only marriages we see are marred by suspicion and violence.
The only utterly clean, uncompromised relationship is between the engine-driver Lantier (Michael Maloney) and his engine, Lison. 'I'm safe with you,' he rhapsodises to her as she speeds along. 'I keep your boilers stoked and you take me to heaven - a perfect marriage.' (Even here, though, you can't help feeling that the engine is being exploited.)
The real difference from Christie, though is in the view of human nature. In the novel The Pale Horse, there is a brief passage on the nature of evil (wisely omitted in Michael Bakewell's dramatisation): 'That's real wickedness. Nothing grand or big - just petty and contemptible.' In Christie's world, evil is a diminution of humanity. But for Lantier - in Sally Hedges' adaptation, the narrator - evil confirms humanity. 'Our inner voice is coated with a thick veneer of civilisation. But the veneer can crack . . . Some would call it demonic possession. Some, madness. Myself, I think it's there in every one of us.'
Where Christie and Zola meet is in the way they use a small community as a microcosm of the big world: Zola has the railway company as his basis; Miss Marple, by contrast, is forever rabbiting on about how observing village life has set her up to understand Life. But for Christie, that was as far as it went - this was her big discovery, the high point of her novels; for Zola, it's just a starting-point. Which is why, in the end, you were glad when The Pale Horse was over; and you can look forward to La Bete Humaine carrying on.