(You might have already found the quote yourself by now but I was literally just re-watching this episode the other day so felt I should share regardless!) The episode is 'The Stable Boy' (episode 18). And here's the quote:
August: "You know, when I get stuck by a block, I usually re-read what I've done, rather than plough ahead blindly. Sometimes I find there'll be a little nugget of inspiration left behind."
Emma: "You mean start over?"
August: "I mean when I start writing, I usually have one idea. And then in the middle I make it another idea and things are different."
Emma: "Your perspective changes."
August: "Exactly."
Not sure how much of the quote you wanted (the converstation still continues on a little after this but that's the main section of it). Hope that helped! :)