Sketching in black and white charcoal on grey paper may be the quickest way I've found yet to capture a scene. My previous favourites have been pen and ink, graphite, or ink and wash (watercolour). Pen and ink is definitely quick (the great advantage being that you can't erase or fiddle with it) but it takes time to add tone to a drawing. Graphite can be quick - an HB pencil, a 2B and an 8B work well - but again it takes time to add the tone. Watercolour can be quick but as I mentioned in 'travel sketch kit for Nepal', I became frustrated with watercolour pans and now use tubes squeezed into a box, which I can't really fit into my pocket.
When sketching animals, you have to be quick, and you can't do much, if any, erasing. My drawings of animals at the zoo are very sketchy, but a reasonable likeness. Also, you can smudge the charcoal for a soft effect. By using grey or another mid-tone paper, you eliminate a lot of 'filling in' and you only have to draw or paint the darks and the lights.
On a visit to Thirlmere railway museum near Picton, NSW, I had an hour to do some sketching and decided to focus on attempting a drawing of one engine - the 1301. Again, with the limited time I wanted to be able to capture the dark parts underneath the locomotive as well as the shiny brass and the brass lamps. In the detail from the drawing, you can see my initial pencil drawing (getting the perspective and relative sizes of the parts of the engine correct).
The paper is Strathmore 'toned gray sketch paper', medium surface 118 gsm. The charcoal pencils are General's brand.