Everytime I do this, I love it and I remind myself to do it more often. People always tell you to sketch people from life but you know how it goes, I rarely do. And it's always a challenge, especially in a museum were people move around change there positions and don't stay in the same place for a long time...The way I do it is that I look at the person, try do remember as many things about the general stature, postition and key features. Then I sketch it quickly and the people are usually gone by then. When I am home I then use these basic sketches and go over them in pen to give them more character and finish them from imagination.
I espcially like the guy on the bottom left ;)