Verbal

Greece has a very high-context orientated culture, which means that the Greeks tend to convey covert and explicit messages which are internalized. Their interactions do not need explanation because of their common history, and they ‘just know’ what is being conveyed to them. Because of this, you might find that the Greeks often interrupt you as they quite quickly find out what it is you are trying to say. Although this can seem rude to many, it is just a common aspect of the Greek way of communication. 

There are two versions of the language: Demotic, which has been used since the 1970s, and Katharevoussa, or the "pure" language.  Demotic is used in governement and media, while Katharevoussa is mostly found in technical publications.  The Greek language is very different from most common languages in that they are very flexible and do not have a rigid word order.

Nonverbal

Personal distance between speakers and friends is much smaller in Greece than many other countries.  This can mean that they tend to stand or sit much closer to you than you might be used to.  Moving away from a person standing too close to you can often be viewed as unfriendly on your part.

Another very important thing to remember is to keep eye contact, as letting your eyes gaze away during a conversation is considered very rude. Touching and standing or sitting close to each other is common.  Furthermore, the Greeks very much like to express themselves and talk out loud among each other. The use of many gestures and facial expressions can sometimes create misunderstandings - e.g. when the Greeks say “no” they tip their head back, which can sometimes be interpreted as a nod. Also, “nay” in Greek means “yes”!

* An important note: using the “thumbs up” is an obscene sexual gesture in some parts of Greece, and a friendly wave with your palm showing and fingers extended is a serious nonverbal insult to a Greek.