The advantage of a fixed form of service is that we know what is coming. Ex tempore public prayer has this difficulty; we don’t know whether we can mentally join in it until we’ve heard it–it might be phony or heretical. We are therefore called upon to carry on a critical and a devotional activity at the same moment: two things hardly compatible. In a fixed form we ought to have ‘gone through the motions’ before in our private prayers; the rigid form really sets our devotions free. I also find the more rigid it is, the easier it is to keep one’s thoughts from straying. Also it prevents getting too completely eaten up by whatever happens to be the preoccupation of the moment (i.e. war, an election, or what not). The permanent shape of Christianity shows through. I don’t see how the ex tempore method can help becoming provincial, and I think it has a great tendency to direct attention to the minister rather than to God.
C. S. Lewis, Letters (1 April 1952) as quoted in A Mind Awake: An Anthology of C. S. Lewis, 147.