It's all reaction at first. The fielder has a lot of computations to make immediately: Can I get to the ball? How fast do I need to run? Is it coming on a line drive or on the ground? Will I need to dive or leap? Where are the other fielders, and would another fielder have an easier play than me? If so, where do I run to get out of the way? Should I cover a base instead?
On plays where more than one fielder can make the play, it's helpful to remember which player might have the easiest play. On a ball hit up the middle, the second baseman should yield to the shortstop, who is running the direction of first base and would have an easier throw because the momentum is taking the shortstop that direction. The same goes for a ball between shortstop and third base. The third baseman has the momentum there. And on a play between second and first, the second baseman typically has the better angle.
