According to Meyer, Schiller was so communicative about whatever he was working on that Goethe knew the plan of Demetrius so well that he probably could have completed it after Schiller's death in 1805. For Goethe, however, a work that had been communicated or discussed with another person was "finished" (erledigt); he wouldn't bother then to write it down.
Goethe's poetic memory was "determined by motif and mood" (motivisch und stimmungsmäßig bestimmt): he carried with him a theme for years, for half a century, reworked it in his mind, somewhat like a lawyer contemplating all possible questions and objections of an opponent and meeting them in advance. Once the matter, the scene, the person, the language had achieved form in his mind (I am paraphrasing Meyer here), then it was finished and had to be communicated. If it was not immediately dictated, Goethe lost the desire to carry it out. In Meyer's view Goethe could conceive a plan for a work, but it was impossible to reconceive it, to begin anew, which can be seen in his reworking of Götz.
Goethe did not like writing by hand. In his autobiography he describes how he began dictating already in his Frankfurt youth, taking advantage of the skills of a young man living in the Goethe family household. In Weimar he dictated his prose and his correspondence to servants, secretaries, friends. This method required immense discipline and concentration and tended to dampen expressions of intimacy. What about inspiration? In my view, Goethe had such "acoustic memory" that what he wanted to say was always at the tip of his tongue, so to speak, waiting to come together with the idea he carried around in his head.
These two views of poets dictating -- Goethe with Johann August Friedrich John (1794-1854), and John Milton with his daughters -- show very different temperaments. The drawing of Goethe and his amanuensis, however, by J.J. Schmeller, is contemporary, while that of the blind Milton dictating to his daughters (now in the New York Public Library) is by the Hungarian artist Mihály Munkácsy ( 1844-1900)