Demosthenes

Nor was this all, but he would make his interviews, conversations, and business with those outside, the foundation and starting point for eager toil. For as soon as he parted from his associates, he would go down into his study, and there would go over his transactions with them in due order, and the arguments used in defence of each course.

And still further, whatever speeches he chanced to hear delivered he would take up by himself and reduce to propositions and periods, and he would introduce all sorts of corrections and changes of expression into the speeches made by others against himself, or, contrariwise, by himself against others. Consequently it was thought that he was not a man of good natural parts, but that his ability and power were the product of toil.

And there would seem to be strong proof of this in the fact that Demosthenes was rarely heard to speak on the spur of the moment, but though the people often called upon him by name as he sat in the assembly, he would not come forward unless he had given thought to the question and was prepared to speak upon it. For this, many of the popular leaders used to rail at him, and Pytheas, in particular, once told him scoffingly that his arguments smelt of lamp wicks.