Brenno had picked out three sketches and asked about different effects. Let's check 'em out!

Basically, Brenno is wondering, when am I making super fine lines to get my halftone, and when am I dry brushing?

Generally, if you see a halftone on this site, 80% of the time it will be from a kind of dry brush/"fan brush" effect that I get from the way I hold the pen and spread the bristles. When I've got a wet brush that hasn't been particularly frizzed, any fanning of the bristles (either by mashing the brush against the paper as with the "fat stroke" or by tipping the brush onto the side of the bristles, á la the "sidewinder") will put a kind of striation in the halftone. You can see this in these two examples Brenno singled out:

Striated brush stroke esp'ly noticeable on the Armco behind the hitchhiker (I mispelled it "armoco" above--sorry!).

Here's an example of fine lines.

Since really fine lines require the brush to be pretty saturated (but not too saturated!), it's hard to get your line razor-fine after you've been drawing for a few minutes (i.e., in the middle of a drawing)--it's almost like your bristles "overheat" as you draw, and need a rest to refill with ink. If you want really fine lines, this "overheating" condition will hold you up, even if your brush tip is pristine. The trade-off to "overheating" is you can now get a pretty subtle dry brush look without actually having to frizz your tip. This is because much of the ink has been brushed out of the bristles, hence the descriptor "dry brush"...but you figured that out already.

So typically I'll start a drawing making certain kinds of marks, (usually the very fine lines for a quick "lay-in"), then as the brush heats/dries, I adjust to more dry-brushy look. I can let the drawing sit a bit, then go back to it for some finishing detail work once the brush has "re-saturated" itself with ink.

Now, all this might sound to you like I'm completely in control while I'm executing any one of these drawings--NOT SO!! I'm usually very rushed, (not least 'cuz I'm trying to capture someone in motion), and very emotional--I see something I want to capture, and all my forethought goes out the window. I forget to plan, I don't react rationally to a changing brush, I get impatient, I mess up....

Basically, these drawings are a chronicle of constantly trying to atone for mistakes. 

Brenno, I hope that helps. Thanks so much for the thoughtful comments!