We all, at some time in our careers, have had to perform in front of an audience without the benefit of a rehearsal or a written part. This can happen for a variety of reasons. You might be asked to sub in a steady band which does not use parts, but which has a tight, well-rehearsed act with a regular drummer. Or you might be part of a pick-up band which has been brought together for a particular job. In either case, if someone requests a number which the band has never rehearsed, the leader might decide that enough people in the band are familiar with the tune and say, “We’ll fake it.” Time is usually taken for quick instructions about key, rhythm, and tempo, and if the end product is not perfect, the audience will appreciate the fact you were trying to please them.