Bauglir said:
Consider this: A rogue with a ring of invisbility manages to move right up behind a victim undetected. He then carefully (and invisibly) reaches around and cuts his victim's throat. The victim is not defending himself in any way, since he is unaware of the rogue's presence. Is this situation a coup de grace?
If not, what is the defining difference that makes the barber scenario a coup de grace, while this is not?
There's quite a big difference. When a person goes to the barber, they don't just happen to be sitting down when a Barber comes out of nowhere and starts to scrape them with a blade- they're
voluntarily putting themselves in a potentially deadly situation. That is, a person walking down the street has a certain expected level of awareness- after all, you've gotta be on your guard in case a car jumps the curb and comes straight at you or something. You aren't going to be completely relaxed except in certain circumstances, and you'll be ready to react to an unexpected situation because, well, nothing is necessarily perfectly safe.
When you go to the barber's, however, you know very well that a person is going to be putting very sharp objects extremely close to vital areas of your body. And you let them. You sit down in that barber's chair, and you voluntarily let them come at you with sharp pointy things. Unless you're the kind of person who jumps every time the Barber starts to cut your hair, you're going to be sitting there, relaxed, and relatively helpless. Much more helpless than, say, a person walking down the street. The person in the Barber's chair is expecting the barber to bring a blade close to his neck- and thus isn't going to react when he sees the barber do so.
In your example, the rogue is still probably going to be able to kill the aforementioned street-walking commoner- they're very Flat-footed, and the Rogue would gets lots o' sneak attack. It isn't the same situation, though.
Anyway, as for making a Sense Motive check, I'd let them do it, but at a HUGE penalty. I know I for one am not "on my guard" or "checking for evil barbers" when I go to get my hair cut. People trust barbers- or else they wouldn't go to them and let them place a sharp blade against their jugular. Take J. K. Simmons' line in Spiderman: The Movie: "I trust my barber."