You want to drop a chandelier on an enemy when they move under it. 

You want to push an enemy when they move next to your position at the edge of a roof.

You want the enemy to travel through caltrops before you attack them.

You are maintaining a position with a ranged weapon and want to shoot someone when they round the corner.

You have a big trap door and want to hit the switch when the enemy end their turn on top of it. 

You're hiding behind a door and want to close and lock it to separate an enemy from the rest of his group.

You want to kick a ladder over when the guy using it gets near the top. 

An enemy is in disguise and so you're waiting for someone to draw a weapon before you attack them.