To achieve a likeness in a painting, the values have to be accurate.Getting accurate values (lights and darks) can be tricky because a perceived value is relative to the other values surrounding it. For instance, when a value is surrounded by darker values, it looks lighter than it actually is. The reverse is also true, lighter values surrounding a value will make it seem darker.
Below is my painting from a weekly figure drawing session that I attend. The first image is before the values were adjusted. You can see that the face feels flat and the eyelids look to prominent.
In the second image, the values depicting the eye sockets and the lower part of her face were darkened and create a more realistic painting. The values on her forehead and cheekbones now look lighter because of the added darker values on her face and in the background.