Is this seat taken?

Your mug of foaming latte on the table before you, you are comfortably "in" the scene you're working on and have been in similar situations at Peet's often enough to know that when someone, seeking your attention, addresses you with the equivalent of "Excuse me," all you have to do is say the equivalent of yes, which may come out as "Go ahead."  The interrupter is politely asking if he or she may take the one or more unoccupied chairs at the table before you.  You do not need to make eye contact for this transaction; it is an expression of minimal (but, importantly, non-hostile) civility, politeness, if you will.  It is the lingua franca of working at a coffee house.  

On the other hand, when you are comfortably "in" the scene and are a court reporter, taking down what the characters say and do, and someone addresses you by your name, without the "Excuse me," or "Pardon me,"  either direct use of name or a prefatory Hi or Hey, then you know eye contact is required.  You also know a different level or degree of communication is under way, a more socially nuanced exchange in which age, gender, status, and degrees of intimacy are factored and responded to.

Consistent with your sometimes mercurial range of responses, you will be pleased, irritated, bored, vastly amused, energized when one of these eye-contact moments arrives, wrenching you from being "in" a scene as you have on occasion been wrenched from the comforts of a splendid dream by the sudden appearance on your chest of one of the two cats who make their way through the warp and weft of your abode.

Further baseline consideration:  there are times when you are not comfortably "in" a scene and welcome eye-contact interruptions.

It comes down to this, writing at coffee shops provides a range of human contact that obviates the need for crossword puzzles, checking email, or inventing comments for Twitter.  The rationale for going to Peet's for an hour or two of "work" is constructed around the matrix of splendid coffee which is offset by a selection of pastries that, regardless of what they are called, taste the same, a dreary fact mitigated by the availability of some packaged noshes of minimal quality.  At the opposite end of the spectrum, which is to say equidistant to the south as Peet's is to the north is La Luna at Colville and Evans in Summerland, where the coffee is so-so but the pastries are from Reynaud's Patisserie, which speaks of flaky crusts or spongy and light layers or fruity preserves that are not drowned in sugar not cooked to the point where their spirit has fled.  There is a forty percent less probability you will be addressed by anyone at LaLuna in which eye contact is required.

Not to forget Xanadu, in the nearby mall featuring a supermarket, pharmacy, dry cleaners, health food store, and barber shop.  It is possible to hide away relatively unseen, but the coffee is undrinkable and the pastries all resemble bicycle seats.

Of course there is always home.