1. It goes in the region with the unit you want to screen.
2.Surpriser has nothing to do with the effect of the Cavalry Screen. During combat the Surpriser increases the chance of units under his command to fire first.
3. Two Surprisers may cancel each other. It's a random number, so one may still surprise and the other not outdo the roll.
Three factors affect game play, hide, detection and evasion. Your units and basically the number of cavalry present affect detection. When you cursor over an enemy stack, you may not get very much info or you may getunit names, commanders and power numbers. Large units. like Corps and Armies may only have a Hide number of 1. A unit with a high detection number will spot them easily. You can see these numbers for your own units. Click on a unit and check out the bottom right-hand corner of the screen. You'll see an icon of a stick figure with an outline behind it and next to this the Hide number. Below this is an icon like a head with a telescope and next to this is the detection number. Below these is a bulls-eye icon with the evasion number. The better the evasion number, the more easily the unit can escape contact with an enemy.
I'm the 51st shade of gray. Eat, pray, Charge!