This delightful anecdote is from Paul Ford (around the 27-minute mark of the first video on this page).
Readers should never pretend to wing it. The worst example I can give you: the feast of Corpus Christi, the reader, a very talented actor in the parish, an Irishman, flew into the sacristy and said "What's the reading today?" They showed it to him, he looked it over, okay. 1st Corinthians. [He said:] "The bread that we break is not the body of Christ. The cup that we bless is not the blood of Christ."
From across the church, I cupped my hands and I said, "Is it not! Is it not!" And when he said, "The word of the Lord," I said "It is not!"