There are lots of warm up sketches that I make in-between deciding what to do next. Some are really silly, some are pretty bad and some become a little bit more involved then intended but all are fun, not taken seriously. Pretty much they were done to keep my hand moving and my mind thinking. I obviously used reference on the sketch of Captain Kirk..
All of these were done in Sketchbook Pro because its pretty easy to sit in front of my monitor and just start to sketch- no set up, no clean up.