Most interesting subject Ruth, been studying lots of paintings from my younger days. I am not good at art, I have not even tried, been wasting my time all my life doing nothing. I try to make a portrait look like a painting. not much work really if you can manage the shadows by asking the person to turn the face and by shifting your position. The source of light is one always, but reflected light comes from various directions. I always prefer diffused indirect daylight. you can diffuse it further by a screen like tissue paper of any kind, Any white semi transparent material, (I use gateway paper used by architects for drawings) the shadows on the face become softer. Light hitting the floor and reflecting on the face give shadows at wrong places which is not good to look. The shadow of the nose falls into the eye. Side light from the Window is fine, the best place is always a portico. If you make the person sit near the end(not AT the end) of portico you get little light from the top and indirect light from the side. To make the shadows a bit brighter, you can ask some one to hold a white cloth or paper, (I use big calenders), as reflector on the opposite side of the source of light. The most important part is the light should be minimum. I understood this by oberving a stray cat which used to visit our house years ago, My room is a favourite place for all the visiting cats, they litter under my bed. The cat looked very beautiful in my room in low light but it looked aggressive in strong light. Its the eyes which make the difference. Any one looks better under low light as the pupils in the eyes are dialated. Large opening of the pupil under low light makes a better portrait, eyes are fully open with no strain of facing the light. First thing you have make sure is that the person's eyes are relaxed. If the person eyes are strained, its better to change the place.