At that point, you no longer have to grind, because your brain has hardwired decision paths that are automatic and produce correct results for extremely complex problems like "is this character likeable and understandable by the reader", and you can effortlessly add a few high value paragraphs to their backstory. But it's foolhardy to think that level of "flow" can be achieved out of thin air, without writing hundreds of characters before them, re-reading them, getting feedback from other readers, studying other briliant authors with a eye for their technique etc.