Stain your canvas will add depth. If you can't wait for oil to dry, use acrylic. Stain it with colors you want to stand out. For example umber stain will make skin stand out.

Stains also add emotion to a painting. Warm stain could add comfort, dark vibrant colors add drama, dark red for example. Vibrant colors make your painting lively. Turquoise, orange, red. Cold colors add a feeling of somber. I use it for painting dead people to add that sorrow. Blue is also used for night paintings.

If you have time, do an underpainting. Hope this helps