Pose your *own* actions/reactions.
Never pose the outcome of an action for another player and try NOT to pose a definite 'hit' from your player. For example:

Do not:
Leroy rears his arm back then swings it forward, slamming it into Henry's face causing his nose to break and blood to splatter everywhere!

Do:
Leroy rears his arm back then swings it forward, aiming for Henry's face with a punch strong enough to break his nose instantly.

This leaves room for the other player to do something. He can either dodge, possibly use his power to block the hit or he could even be shoved out of the way by someone else. This is his/her character and he/she is the one who has the right to pose what happens to them, not anyone else.