As for the "click of the gun" debate on bear responses, I personally believe that it is the combination of unfamiliar visual stimulus (upright human), unfamiliar olfactory stimulus (human smell) and unfamiliar aural stimulae (human screeching like a banshee and/or a clicking noise) is enough to make a most animals stop and think " 'ang-on-a-minute... wossis-all-about?" and go away, unless they have a big enough stimulus pushing them forward such as protecting young...
Scream all you want, clicking the action or simply pulling your gun out of the holster and having a bear simply run off most likely won't happen like Skidder said he has had happen to him. Maybe it did happen, but if a bear has decided to go after you it's going to take much more than a clicking action or just the removal of a gun from the holster to make him change his/her mind. It's what happens after you pull the gun out and click the action that makes the difference. Let's be honest here, on this forum, and all others most of us have not been charged, attacked or chewed on by a bear, just a lot of "what if" scenarios posted everywhere.