The Screwtape Letters is itself a wonderful argument for the positive Christian use of comedy. In fact I once heard John Cleese read the book on tape - hilarity itself!
Lewis' book consists of the letters of Screwtape, a senior demon, to his nephew Wormwood. It's advice on how to really capture a soul for evil.
Here is letter 12 on humour. (NB: of course for Screwtape, "our Father" is the devil. "The Enemy" is God).
MY DEAR WORMWOOD,
Everything is clearly going very well. am specially glad to hear that the two new friends have now made him acquainted with their whole set. All these, as I find from the record office, are thoroughly reliable people; steady, consistent scoffers and worldlings who without any spectacular crimes are progressing quietly and comfortably towards our Father's house. You speak of their being great laughers. I trust this does not mean that you are under the impression that laughter as such is always in our favour. The point is worth some attention.
I divide the causes of human laughter into Joy, Fun, the Joke Proper, and Flippancy. You will see the first among friends and lovers reunited on the eve of a holiday. Among adults some pretext in the way of Jokes is usually provided, but the facility with which the smallest witticisms produce laughter at such a time shows that they are not the real cause. What that real cause is we do not know. Something like it is expressed in much of that detestable art which the humans call Music, and something like it occurs in Heaven-a meaningless acceleration in the rhythm of celestial experience, quite opaque to us. Laughter of this kind does us no good and should always be discouraged. Besides, the phenomenon is of itself disgusting and a direct insult to the realism, dignity, and austerity of Hell.
Fun is closely related to Joy-a sort of emotional froth arising from the play instinct. It is very little use to us. It can sometimes be used, of course, to divert humans from something else which the Enemy would like them to be feeling or doing: but in itself it has wholly undesirable tendencies; it promotes charity, courage, contentment, and many other evils.
The Joke Proper, which turns on sudden perception of incongruity, is a much more promising field. I am not thinking primarily of indecent or bawdy humour, which, though much relied upon by second-rate tempters, is often disappointing in its results. The truth is that humans are pretty clearly divided on this matter into two classes. There are some to whom "no passion is as serious as lust" and for whom an indecent story ceases to produce lasciviousness precisely in so far as it becomes funny: there are others in whom laughter and lust are excited at the same moment and by the same things. The first sort joke about sex because it gives rise to many incongruities: the second cultivate incongruities because they afford a pretext for talking about sex. If your man is of the first type, bawdy humour will not help you-I shall never forget the hours which I wasted (hours to me of unbearable tedium) with one of my early patients in bars and smoking-rooms before I learned this rule. Find out which group the patient belongs to-and see that he does not find out.
The real use of Jokes or Humour is in quite a different direction, and it is specially promising among the English who take their "sense of humour" so seriously that a deficiency in this sense is almost the only deficiency at which they feel shame. Humour is for them the all-consoling and (mark this) the all-excusing, grace of life. Hence it is invaluable as a means of destroying shame. If a man simply lets others pay for him, he is "mean"; if he boasts of it in a jocular manner and twits his fellows with having been scored off, he is no longer "mean" but a comical fellow. Mere cowardice is shameful; cowardice boasted of with humorous exaggerations and grotesque gestures can passed off as funny. Cruelty is shameful-unless the cruel man can represent it as a practical joke. A thousand bawdy, or even blasphemous, jokes do not help towards a man's damnation so much as his discovery that almost anything he wants to do can be done, not only without the disapproval but with the admiration of his fellows, if only it can get itself treated as a Joke. And this temptation can be almost entirely hidden from your patient by that English seriousness about Humour. Any suggestion that there might be too much of it can be represented to him as "Puritanical" or as betraying a "lack of humour".
But flippancy is the best of all. In the first place it is very economical. Only a clever human can make a real Joke about virtue, or indeed about anything else; any of them can be trained to talk as if virtue were funny. Among flippant people the Joke is always assumed to have been made. No one actually makes it; but every serious subject is discussed in a manner which implies that they have already found a ridiculous side to it. If prolonged, the habit of Flippancy builds up around a man the finest armour-plating against the Enemy that I know, and it is quite free from the dangers inherent in the other sources of laughter. It is a thousand miles away from joy it deadens, instead of sharpening, the intellect; and it excites no affection between those who practice it,
Your affectionate uncle