Auerbach's heads of Julia, unlike those of JYM, are painted using acrylic paint, a medium that is not only less messy but also faster drying. The choice of medium also explains the flatter surfaces of these paintings.
Although over the years, the range of poses used by Auerbach's models have changed, it is broadly the case that whilst in the 1980s JYM was frequently depicted seated in a wooden chair, in the 1990s Julia frequently lies down, hence the number of reclining heads, in which she appears to stare upwards.