What it is, in fact, is a technical solution to the drama's overriding problem, that of translating a private mental drama into a public one; Henry has to read aloud because otherwise we wouldn't know what he was looking at. One wouldn't normally want tobe prissy about such conventions except that the moment is also emblematic, the clearest explanation of why a consistently funny book has become only a sporadically funny television film.
In the novel, the best joke is the mismatch between Henry's generally submissive manner and the seething heat of his private thoughts. It's appropriate that his daughter is studying volcanoes - Henry is apparently dormant but there are danger signs - smoking fumaroles of resentment, sudden molten splutters of sarcasm. The explosion is to be a silent one - the poisoning of his ghastly wife, a woman so naggingly insistent on his failure to communicate that she doesn't hear anything he says to her. On paper there's no difficulty - the outside world intrudes on the real action, which is Henry's inner turmoil. On film, though, the outside world (the visible world) automatically takes precedence and private reveries have to be amplified to be heard.
Short of adding subtitles, Nigel Williams's adaptation of his own novel tries everything - voiceover, muttered asides, bits of dialogue, direct address to camera. But almost every method has its costs; during voiceover Robert Lindsay has to mug crudely, to mark time with his thoughts; what was caustic and funny as silent commentary, precisely because it was at odds with what it is permissable to say, becomes slightly unconvincing as spoken dialogue; above all Henry's character changes - he isn't a secret only we know but an uneasy hybrid of confident scorn and social terror, switching from one to the other as the plot requires it.
It isn't that television necessarily coarsens everything it touches - Porterhouse Blue provides a fairly recent example of a comedy that actually became more subtle on screen, because the actors had to add character to what was essentially slapstick - but the subtleties of The Wimbledon Poisoner have a much harder time of it - they have to put on weight in order to show. When Henry is shopping for chicken - thoughts of murder running in his head - he ends up gnashing into the shrink-wrapped bird. He hasto do this in order to be demonstrably odd when interrupted by Russell Rush, the local detective inspector, but the essential falsity of the thing makes it virtually impossible to laugh. I found myself worrying about botulism instead, wonder ing whetherLindsay had the chicken swabbed with disinfectant before undertaking this dangerous stunt.
A lot of the jokes survive, particularly the wilder inventions, (such as the optician who has to talk one of his patients down the High Street on her mobile phone after she loses her contact lenses). But it is, tellingly, at its funniest when the collision between public statement and private thought is preserved - as it is during Henry's funeral address for his first victim, the local doctor who accidentally hogs down a thallium-basted chicken breast. "No human life form - not even Donald Templeton - is completely beneath contempt," declares Henry. What oft was thought, in other words, but ne'er said aloud.