One of the most fascinating things I've read by Stevenson was his description of his creative process, the success of which he attributed to the "little people" in his dreams, or what he called his "Brownies".

Referring to himself in the third person, Stevenson writes:

"This honest fellow had long been in the custom
of setting himself to sleep with tales, and so had his father before him; but these
were irresponsible inventions, told for the teller's pleasure, with no eye to the
crass public or the thwart reviewer: tales where a thread might be dropped, or one
adventure quitted for another, on fancy's least suggestion. So that the little
people who manage man's internal theatre had not as yet received a very rigorous
training; and played upon their stage like children who should have slipped into the
house and found it empty, rather than like drilled actors performing a set piece to
a huge hall of faces.

"But presently my dreamer began to turn his former amusement of
story-telling to (what is called) account; by which I mean that he began to write
and sell his tales. Here was he, and here were the little people who did that part
of his business, in quite new conditions.

"The stories must now be trimmed and pared
and set upon all fours, they must run from a beginning to an end and fit (after a
manner) with the laws of life; the pleasure, in one word, had become a business; and
that not only for the dreamer, but for the little people of his theatre. These
understood the change as well as he. When he lay down to prepare himself for sleep,
he no longer sought amusement, but printable and profitable tales; and after he had
dozed off in his box-seat, his little people continued their evolutions with the
same mercantile designs.

"All other forms of dream deserted him but two: he still
occasionally reads the most delightful books, he still visits at times the most
delightful places; and it is perhaps worthy of note that to these same places, and
to one in particular, he returns at intervals of months and years, finding new
field-paths, visiting new neighbours, beholding that happy valley under new effects
of noon and dawn and sunset. But all the rest of the family of visions is quite lost
to him: the common, mangled version of yesterday's affairs, the
raw-head-and-bloody-bones nightmare, rumoured to be the child of toasted cheese --
these and their like are gone; and, for the most part, whether awake or asleep, he
is simply occupied -- he or his little people -- in consciously making stories for
the market.

"This dreamer (like many other persons) has encountered some trifling
vicissitudes of fortune. When the bank begins to send letters and the butcher to
linger at the back gate, he sets to belabouring his brains after a story, for that
is his readiest money-winner; and, behold! at once the little people begin to bestir
themselves in the same quest, and labour all night long, and all night long set
before him truncheons of tales upon their lighted theatre.

"No fear of his being
frightened now; the flying heart and the frozen scalp are things by-gone; applause,
growing applause, growing interest, growing exultation in his own cleverness (for he
takes all the credit), and at last a jubilant leap to wakefulness, with the cry, "I
have it, that'll do!" upon his lips: with such and similar emotions he sits at these
nocturnal dramas, with such outbreaks, like Claudius in the play, he scatters the
performance in the midst.

"Often enough the waking is a disappointment: he has been
too deep asleep, as I explain the thing; drowsiness has gained his little people,
they have gone stumbling and maundering through their parts; and the play, to the
awakened mind, is seen to be a tissue of absurdities. And yet how often have these
sleepless Brownies done him honest service, and given him, as he sat idly taking his
pleasure in the boxes, better tales than he could fashion for himself."

...

"Who are they, then? and who is the dreamer?

"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 farther 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."

http://www.worlddreambank.org/S/STEVBROW.HTM