Most missions in GTA5 have optional mission objectives. Completing them or not determines what medal you receive at the end of the mission (gold, silver, bronze).
So far I have found it quite frustrating to complete a mission only to be told that I didn't get gold because I didn't "headshot 10 bikers" or because I "wrecked my vehicle".
Is it possible to view the optional objectives during a mission? I've had a quick look around on the in-game menus, but haven't found anything so far.