Hmm - he does look kind of like Don Knotts. I was trying to make a guy who looks tougher, but oh well.

Hmm, if you're going for tougher, I think first thing you might wanna do is, get rid of some of the 'aged' definition in his face. You'd especially wanna lighten up on the wrinkling below and around the eyes, and the lines going all the way around the mouth (mostly that bit under the mouth). Another tip to making a 'tough' looking character would be to go for a more square-er face, especially in the jaws (widen them and making the angles of the jaw, sharper).
Hopefully this will help some :-/...

Thanks Gigandas - this is just a construction schematic, so the contour lines are there not to add character but just for me to establish a feel for the topology. By tougher I just mean not necessarily comedic or prone to caricature, as Don Knotts certainly is (not necessarily a tough character, but just some head with a very prominent skull which doesn't have any really exaggerated features). I'm hoping to find some way to establish the base for a skull-head from which I can get some sense of how to draw heads at different angles, but a Don Knotts head is kind of an odd one to start out with. I'd appreciate any help along the way with respect to anatomical correctness and perspective (digital alterations appreciated). The faster I can construct them without significant errors, the better.

I agree with Gig about tough guys often having a squareness of the face, that kind of hulking-brute look. But I think that wrinkles and such don't detract from a person looking tough; done a certain way, wrinkles and other imperfections can make a guy look wizened and tough. Clint Eastwood has had wrinkles for quite some time but with that squint of his he still looks tough as nails.

I'm really impressed by your dedication, comd. If you don't end up improving by leaps and bounds (good as you are already) I think it'd be some sort of cosmic crime. I can't really give you any advice on anatomy or such, but you might try asking Cloxboy about that - he draws tons of faces at all sorts of angles without references. IIRC, he did tons of anatomy drawings back in the day, so he might be able to guide you along the path to anatomical enlightenment. ;)