I've only just bumped into this video a couple of days ago and I've just watched through it the once. Jack "King" Kirby is seen executing a now quite well known character drawing, that of Doctor Doom behind the mask. It's interesting for a number of reasons, the weight of line he achieves while ,I believe, not pausing to hone his pencil, probably because he's using a clutch pencil and the scale at which he's working. He's also got a mobile arm but that I mean he doesn't tilt the paper much to orientate his line work, that's something pros pick up after a lot of practice. Clutch pencils are a boon to the pro, I myself didn't cotton on to them for a while, once I did, my productivity increased by about a 1/3 so it's no surprise that's what Kirby should be using.
There's a still of an ink rendering of this drawing at the end of this video, it's quite a small image but it does seem a nice job, even so, it's possible to discern some quite noticeable differences between Mike Royer's sublime ink job of the same drawing, a nice illustration of the importance of inking.