[JRutledge]

You can always set a screen if both players are moving in the same direction. Or you can move on a screen anytime as long as there is no contact. Once there is contact the screener in the last example is responsible for the contact.

That being said I try to call fouls on illegal screens when the defender or screened player contact altered or stopped the player. If they just give up because someone is in front of them and the barely touch, that is not good enough.

Peace