The long passage on the nature of art is certainly didactic, but also much more graceful than similar such passages by Tolstoy and Mann. The voice is nominally that of Marcel/narrator, but the author’s voice is dominant. For instance, consider this statement on the bearing of real-life people to characters in the novel:
By such tones of voice, such variations in the physiognomy, seen perhaps in his earliest childhood, has the life of other people been represented for him and when, later, he becomes a writer, it is from these observations that he composes his human figures, grafting on to a movement of the shoulders common to a number of people–a movement as truthfully delineated as though it had been recorded in an anatomist’s note-book, though the truth which he uses it to express is of a psychological order–a movement of the neck made by someone else, each of many individuals having posed for a moment as his model. (VI,306)
But the novel we are reading is in the form of a memoir, where the lead character learns from his experiences with the people of his life how to become a writer, so the characters cannot be composites of others. Unless, that is, this is Proust, the author, having his say. At the same time, nonetheless, it is a warning not to look for the author and his friends in this novel.