Okay back to it...
After I rough in the composition, I photograph it and save it to project back onto the canvas surface later.
For this painting I now want to put in a gradient, tonal underpainting. I don't usually do this for smaller paintings, instead I move around the canvas, painting in cool and warm tones as I go. But with this larger one, I think I'll be able to focus more on working out space and value relationships between the figures, if I take care of some of that with the background now.