It might be forgotten these days that C.S.Lewis wrote more than the Narnia stories. His series of letters from the older tempter Screwtape to his nephew Wormwood, a devil still in training, have surfaced at the Courtyard Studio as an unusual pre-Christmas treat. And according to the Canberra Theatre website the season is a sell out, so it is clear that his non-Narnian work still attracts.

This production does not quite find a theatrical idiom to catch everything in Lewis' robust and theologically fascinating writing. Yannick Lawry's Screwtape is rather too gentlemanly but there's some nicely inventive comedy from George Zhao as Screwtape's general factotum Toadpipe.

In front of a dramatic backdrop of what look like clouds but are more likely to be smoke from the everlasting fires there is a kind of office cluttered with dark furniture and books where Screwtape conducts a correspondence with Wormwood via an infernal postal service that involves thunder and a lot of guessing as to where the black enveloped replies are when they arrive. Wormwood is luring a young man into damnation while his uncle coaches, often impatiently, from the sidelines.

Screwtape is attended by the obsequious Toadpipe who is servant, secretary and eternal yes-man. Occasionally Zhao's expressive and crouching under-demon uncoils to illustrate a point on a blackboard or morphs briefly and effectively into one of the human characters in the story. Sometimes he simply demonstrates the subtly negative behaviours that Wormwood should encourage if the young human is going to achieve damnation.

Hell certainly can be about tedium but there is a real difference between the hellish theatrical tedium set up by the likes of Samuel Beckett or Harold Pinter or Jean-Paul Sartre and what happens here. The repeated format of the posting and the arrival of the letters quickly palls despite some attempts at variation, possibly because it needs to build more intensity. And the updating that removes the tensions of World War II air raids from the ultimate fate of the young man waters down the original denouement.

Lewis' road to damnation is depicted as a gentle one, paved with all kinds of good intentions that come to nothing, rather than dramatic evildoing, and that is part of its strength. But there's still a looking back here to older forms of theatrical devils, like those in medieval religious drama or Marlowe's Doctor Faustus or even Shakespeare's Richard III and they are anything but domestic in their theatrical impact. This show could use a less tentative approach and a more dangerous Screwtape.

However, a packed out audience was clearly absorbed by the chance to see Lewis' devils and their arguments realised on stage.