Stevenson writes in his A Chapter on Dreams, which you can see a book scan at that link, and a text version at Project Gutenburg:
Well, as regards the dreamer, I can answer that, for he is no less a person than myself;—as I might have told you from the beginning, only that the critics murmur over my consistent egotism;—and as I am positively forced to tell you now, or I could advance but little further with my story. And for the Little People, what shall I say they are but just my Brownies, God bless them! who do one-half my work for me while I am fast asleep, and in all human likelihood, do the rest for me as well, when I am wide awake and fondly suppose I do it for myself. That part which is done while I am sleeping is the Brownies’ part beyond contention; but that which is done when I am up and about is by no means necessarily mine, since all goes to show the Brownies have a hand in it even then. Here is a doubt that much concerns my conscience. For myself—what I call I, my conscious ego, the denizen of the pineal gland unless he has changed his residence since Descartes, the man with the conscience and the variable bank-account, the man with the hat and the boots, and the privilege of voting and not carrying his candidate at the general elections—I am sometimes tempted to suppose is no story-teller at all, but a creature as matter of fact as any cheesemonger or any cheese, and a realist bemired up to the ears in actuality; so that, by that account, the whole of my published fiction should be the single-handed product of some Brownie, some Familiar, some unseen collaborator, whom I keep locked in a back garret, while I get all the praise and he but a share (which I cannot prevent him getting) of the pudding. I am an excellent adviser, something like Molière’s servant. I pull back and I cut down; and I dress the whole in the best words and sentences that I can find and make; I hold the pen, too; and I do the sitting at the table, 188 which is about the worst of it; and when all is done, I make up the manuscript and pay for the registration; so that, on the whole, I have some claim to share, though not so largely as I do, in the profits of our common enterprise.
Stevenson, A Chapter on Dreams (emphasis added)
(I found this by Googling
stevenson "Brownies", finding this British Library page, looking at the scan, seeing the quote I mentioned above, then Googling
stevenson a chapter on dreams and finding a link to the Project Gutenburg page.)