It makes an enormous difference if you are visiting a touristic place or one that is not, and also if you speak Greek or not. Tourists are led to believe that all Greeks now know English, but this is not true if you visit a place off the beaten track, where a native might go look for some school children to help translate, since people over 35 of age were born before English became a required language in the public schools. In any case, most Greeks, if approached respectfully tend to be polite and helpful, and teenagers are mostly very polite, and happy to practice their English at the same time as offering assistance.
It cannot be overstressed, however, that tourists will see mostly the polite surface of 'Greekness' in places where foreign visitors are a large presence. Many Greeks in such places have drawn defensively into themselves when not forced to put on the public smile and carefully studied language required by their stressful, and poorly paid jobs. Probably the best way for anyone who hasn't come to live in Greece (and with a serious interest in learning the language and penetrating deeply into the culture), is to observe how Greeks are when among themselves. One of the best venues for this is the traditional Greek taverna or the sidewalk cafes (not the tourist places, where you'll see very little of the genuine article).
The behavior you will see in these places is very 'Greek', though it also very 'Mediterranean' (and very Anatolian). So we find ourselves back in shared cultural territory. It has been said that eating is the national Greek pastime, and indeed this is true. Eating and socializing go very hand in hand, and the Greeks are a very social and animated people. They tend to talk loud, talk with their hands (even when having a conversation on a cell phone), laugh a lot, and be much more physical with each other than north Europeans (or North Americans).
One of the salient differences in this realm seems to be very related to the intensely close knit family in Greek culture and to the consequent lack of equating of physical expression of affection with sex. This needs some elucidation. Perhaps one of the things that attracts many foreigners to Greece is the sense of being among people who are very comfortable with themselves and each other, and this comes from the deep sense of family.