The fastest way is usually to drop step. This requires pushing off on the outside leg and dropping the hitting side leg under the center of balance. If you can't reach the ball by this, then a cross-over is going to be fastest.
With the volley, the most important thing is going to be to get the ball back over if someone is trying to pass you. If you've already forced a weak reply on the approach, then choose the footwork that allows you to hit it with control and pace.