"Draw lines, young man, and still more lines, both from life and from memory, and you will become a good artist." Ingres advice to a young Degas.
At the High School of Art and Design Irwin Greenberg advised his students to draw every day. Till this day I strive to adhere to that advice, drawing from imagination, from life, from photo reference.
Sometimes I would pause my TV and draw one of the actors on the screen or I will read a book and try and visualize the characters and scenes and sketch those out on paper. My preference is to draw from life but sometimes a model is not readily available so I prop a mirror on a chair so I can do a self portrait or a just sketch away using my imagination.
Drawing the face has always been more interesting to me than anything else so I often fill up pages with different heads. In the down time, between paintings, when I am wondering what to do next or trying to build up to tackling a large painting I make these sketches.