Funny one this. Very early on I thought I'd nailed the concept for my 'haunt' illustration - a lone walker in the snow on a moonlit night, with two lines of footprints behind him. I wanted to play on that particular kind of isolation to be found in the dark, the cold, the frozen north (captured nicely in the novel Dark Matter: A Ghost Story ) and the eeriness of being alone and not being alone. Sorted.
Except... I just couldn't get it. While I liked the idea, all of my sketches trying to compose the drawing failed miserably. Either the walker was too far away for you to care about, or too close to get a tangible sense of isolation. Tsk! So I came up with the above almost on a whim, working with the thought of being haunted inside your mind and externalising it. Thanks to one day's commute and the Sketchbook Mobile app (whose range of brushes made the picture) I had my haunted drawing, without a single spook in sight.