My mother reading the christmas card list and my dad holding two tomatoes. I tried drawing my mother before christmas, got her to sit for me for about an hour and yet it was impossible to draw her then. She was too self conscious and put up an unconscious barrier made of her thoughts- or she just wouldn't relax (all her fault you notice). This time she was unaware and it was much easier to draw her likeness. I was just looking at the portraits I did at that market and they are terrible. People were talking to me as I worked and I was on the spot performing on demand. This doesn't produce good art. How particular it all is.
Now that I mentioned it I could post some portraits I did of my father which are quite nice from when we were on a train journey together to London and the ones I refer to of my mother which are terrible if you mean likeness wise but still they capture something, as all drawings do. (If you click read more you can see)
The point is you can't do it well every time as I need to be feeling very calm and balanced. I wonder if this is universal to artists, surely it is. As life is a sine wave not a flat line, I am not consistently drawing things I like; most of the time it's an exercise. The less you worry about it the better though, I find, just do what you do. Anyway the things you do straight off without too much preparation are often the nicest. Or maybe my dad is just easier to draw than my mum, I am just not sure why perhaps he doesn't mind the feeling of being observed.