is that the primary defender does not have to be set, just in legal guarding position. However, the secondary defender needs to be set--meaning he cannot be sliding laterally but may be moving vertically. So if the on-ball defender beats the ball handler to the spot, but is still moving it could be a charge. But the second defender stepping into the lane cannot still be moving to the side.

Whether I read this in a rulebook or just heard from an official, I can't remember, but thats what I used as a ref.

Also there has to be contact for any foul--just because the defender is there in time and then falls down does not make it a charge