'[...] the distinctiveness of radio is not that it involves the imagination while the other media do not, but that it involves it to a different extent. In literature everything must be imagined since nothing can be seen except printed words, nor can anything be heard. In the visual media many things can be seen and proportionately less is left to the imagination. In radio many things can be heard, and this direct intimation of the material world is perhaps why, in its drama productions at least, its verbal descriptions of a physical setting or of a person's thoughts or appearance are generally much more economical than those of literature and closer to those of theatre, film and television. Moreover the fact that its codes are auditory and therefore exist in time explains the greater sense of 'liveness' that we get from radio 9and the visual media) than we do from literature; for when we start to read a book we know that the last page has already been written. But radio, even when its programmes are pre-recorded, seems to be a "present-tense" medium, offering experiences whose outcome lies in an unknown future. Like theatre, film and television, then, it seems to be an account of what is happening rather than a record of what has happened. But the fact that nothing can be seen on the medium means that the demands which it makes upon the imagination are much greater than those made by the visual media and almost as great as those made by literature.'
Andrew Crissel, Understanding Radio, publisher: Routledge