What if my character uses a ranged weapon, such as a longbow, crossbow, shortbow, etc. and the enemy isn't aware of an incoming arrow/bolt? Do they automatically gain awareness, deflect the arrow/bolt and charge down the field while my character waits to be countered, checking the local news on his/her iPhone? 
What I'm asking is about ranged bow attacks and sneak attacks. How are they implemented? What are the chances of a sneak attack being successful, and how does a sneak attack differ from a normal, face to face attack?