Using Stealth in Combat
A Thief may be able to accomplish a back-stab attack in a given round in the following manner.
Declare the intent to sneak silently (if they might be heard) and hide in shadows (if they might be seen). Then move into the desired position, which must be within normal movement range, but may move past enemies. Finally, the Thief rolls his check(s).
If the check(s) succeed, the Thief may back-stab the target.
If the check(s) fail, an attack of opportunity may occur if it had been possible during movement, and the Thief may choose to end movement anywhere from the attack to the final position. Regardless, there is no opportunity to back-stab.