Overwatch Triggering

[mookiefer]

I'm a little confused as to what can trigger an overwatch attack. Is it only movement or any type of activation?
If my opponent doesn't move (aims) and just shoots can I fire my overwatch unit before he does or at all?
Can you choose not to activate a unit so that your opponent with a unit on overwatch doesn't get to shoot at it?

[inrepose]

It was intended to be used when an enemy unit advances either in your units field of view, entering or exiting your LOS for that unit (even if they were out of range). However you could house rule an ability to shoot back as a reaction to an Aim and Fire. I think in that instance it would be fair to count fire as generally a simultaneous fire fight. So both roll the attack following any adjustments for overwatch and count wax'd models after you complete all fire for both groups/vehicles firing.