In rhetoric, a dead metaphor is one that has lost its quality of joining two objects. It is a cliché. You grasp the concept. We use them socially out of habit, laziness or not caring for the person to whom we are talking. Marcel realizes that he will have to look within to keep language fresh in his writing.
Above all I should have to be on my guard against those phrases which are chosen rather by the lips than by the mind, those humorous phrases such as we utter in conversation and continue at the end of a long conversation with other people to address, factitiously, to ourselves although they merely fill our mind with lies–those, so to speak, purely physical remarks, which, in the writer who stoops so low as to transcribe them, are accompanied always by, for instance, the little smile, the little grimace which at every turn disfigures the spoken phrase of a Sainte-Beuve, whereas real books should be the offspring not of daylight and casual talk but of darkness and silence. (VI,302)
Tags: Proust cliche