If you do a 2AP dash, you get one extra tile of movement than if you do a 1AP move, then move again with your second AP.
Moving 'through' one of your survivors can sometimes show that it's going to cost 2AP, but will then only cost 1AP. For example, you have two survivors next to each other. You want to move to the left survivor to the tile on the right of the right survivor. This costs 1ap and says as much. However, if you have two survivors standing at a diagonal from each other, and attempt to move through that survivor to the tile on the diagonal on the far side of the second survivor it will show it as a 2AP move. BUT it actually will only cost 1AP.
Walkers can move two tiles horizontally or vertically, but they can move three tiles on a direct angle. This commonly confuses people.