Sketching with a 1 pixel brush in Photoshop. The focus was, in priority order: 1. Capture a likeness/spirit of the character. 2. Use a single line to convey a surface/part of the subject in space. 3. Use as few lines as possible to convey those ideas. 4. Don't erase/use undo. The photos were found on http://www.humansofnewyork.com/, an awesome resource for character inspiration.