For those interested in a translation from an aerospace engineer (translation/commentary in italics):
EM: Grid fins are stowed on ascent and then deploy on reentry for "x-wing" style control. What you're looking at is called a grid fin. It's only used during reentry, the rest of the time it's folded against the rocket body and out of the way. When they deploy the four fins create a kind of "X" shape. I like star wars. Each fin moves independently for pitch/yaw/roll. Each fin is controlled separately, meaning depending on the way they are used together, we can impart pitching forces, yawing forces, or rolling forces on the vehicle.
JC: Good luck. If it doesn't work I get to say "I told you so". We had supersonic control inversion issues with actuated fins, went back to little thrusters that worked at all speeds. We tried that. They only work in certain speed ranges, and sometimes when you go from one speed regime to another (aka slowing down) the control forces imparted by the fins will reverse (aka invert, aka control engineer's nightmare)! We decided to stick with thrusters because they always push in the direction you tell them too, no matter the speed! Also our control engineers said they'd quit otherwise.
EM: No choice. We thought of that. I don't take threats from my engineers. Entry velocity too high for a precision landing with N2 thrusters alone. We're going so fast when we come in that little thrusters with nitrogen (N2) propellant aren't strong enough (or we don't want to carry all the nitrogen fuel we would need). Must have aero surfaces for pitch trim. Aerodynamic surfaces (aka fins) don't cost fuel. We need a lot of force to control the pitch of the rocket (which way it's pointing) when it's coming back in. Using thrusters instead would cost us way too much mass in fuel and big thrusters, and break the design (aka MUST use something else).
JC: I don't disagree, but I'm concerned. Trying to be polite. Maybe offset CG or static trim tab for a touch of body lift, then roll it? Getting technical. What if you tried this: Tweak where your center of gravity (CG) is (by shifting around where heavy stuff is inside the rocket aka ballast mass), as well as the aerodynamics of your body by itself (static trim), until the rocket generates a small amount of lift by itself (meaning as it's falling back to earth air passing around it will tend to push it in one direction aka "lift" [<- in quotes because it's not so much up as sideways in this case]), then roll it (pitch is hard to control, but roll is easy, so then you just roll the vehicle until the "lift" is in the direction you want).
EM: That works for Dragon, but hard to do for something long like Falcon. Not a bad idea, that's how it's done with basically all entry vehicles (think Apollo, Orion, Mars landers, Dragon), but it's hard to generate enough lift force using a falling tube.
Really a lot going on here. I always surprised people try to have these types of conversations on twitter.