In her own words...
Robert Henry Rubin: Do you sketch before you paint?
Dorothea Tanning: I sketch if I am about to engage in an important painting. I like to make sketches first and sometimes like to capture some of the spirit of the sketch when I make the painting. On the other hand, sometimes I sketch on the canvas and then might decide to change it all. That's what is still exciting about painting.
—from interview with Robert Henry Rubin, "Dorothea Tanning," NIGHT (November 1979).