Usually I have an overall idea, do a kind of straight-ahead pass for the key poses (but with a general plan in mind), and finish up by inbetweening those key poses. So, both?
This one, however, was pretty much straight ahead as it was just a doodle. I had no idea what I was going to do until I did it. I did go back and add some inbetweens to smooth out and improve what I had at the end, though.
Kinda. If I were to have (for instance) a scene where I knew I had to have the main character run over to a gun, pick it up, and take cover behind a barricade, I'd sketch out a rough BG first and do some planning. Here's where the guy starts, here's the gun on the ground, and here's the barricade. I'd then try to visualize how the scene might play out. Is the guy experienced enough that the gun practically gives itself to him, or does he have to fumble around with it before he gets a decent grip? Does he trip and have to drag himself the last couple feet to the barricade, or does he slam his back against it before looking over to see what's ahead? This is where acting it out might first come in useful.
Often what I do is straight-ahead the poses, but in thumbnail form. I figure them out super-rough all on the same sketch page, because seeing them all together kinda helps me see what I'm doing. Then I'd probably open Flash. From there, I'd probably be doing a weird hybrid of pose-to-pose and straight ahead, keeping what I liked from my thumbnails while allowing for happy accidents.
Of course, that's if I had an actual assigned scene, or at least had a clear goal before I started. With silly stuff like this, though, I just doodle a character and go. "I drew him meditating, what next? What if he does a backflip instead of just standing up? And hey, let's turn that into a handspring out of frame, just for fun. Playing that back, he looks pretty exuberant, so let's color him like Michelangelo." Like that. X)
Yeah that all makes sense, I tend to do the same with the page-doodle thing so I can see my timeline and what may happen throughout it, get an idea of the weight and such. But yeah I've never really tried allowing hapsidnents before; think I'll give it a bash sometime ^^