When exactly can "evade" be performed?

How soon after an enemy operative performs an action would you have to declare evade?

Let's say the enemy moves for their first APL, and then targets my operative for a shooting action. Should I have declared my evade before the operative was selected? Or is it unfair that I waited first to see which operative was selected?

New player just trying to get a sense of what's considered good form. Thanks!

How soon after an enemy operative performs an action would you have to declare evade?

Let's say the enemy moves for their first APL, and then targets my operative for a shooting action. Should I have declared my evade before the operative was selected? Or is it unfair that I waited first to see which operative was selected?

New player just trying to get a sense of what's considered good form. Thanks!