What was Greek art characterized by in the Hellenistic period?

Hellenistic Art was from 500 years after the Golden Age of Greece (or Hellenic period). The Hellenistic period occurred when the Roman Empire controlled the eastern Mediterranean, including Greece, during the 1st and 2nd centuries A.D.

The art of the Hellenistic age was very different from the previous periods of Greek Art (Archaism and Classicism). Hellenism was not only an art movement, but an entire period of Greek civilization. Alexander the great had conquered an empire and brought together peoples from all over the world. Due to the massive size of the lands, royal iconography became a large part of the art world. Images of Alexander the Great could be found everywhere, and images of other rulers would be placed on coins. Many leaders tried to emulate Alexander by making their portraits look like his. Lysippos, a very famous sculptor of the time, was the primary sculptor and creator of Alexander's image. The Classical age was not marked by its reverence of the image of the rulers. The artists during this time also took a greater interest in the personality and the psychology of the portraits, unlike the more stoic and stylized portraits of the classical period. The faces took on more emotion during the Hellenistic period and sometimes attributes were added to exaggerate the personality or the profession of the individual. The artists used a persons inner qualities to adjust their portraits. For example, a general may be given severe features to show strength and a philosopher might receive exaggerated wrinkles to show his intelligence. This period also marked a new interest in the drama that wasn't seen before. Sculptures frequently broke the plane of their space, and their faces were incredibly emotive. There are sculptures that show incredibly dramatic moments which are full of movement. Realism and worldliness was important too. Some sculptors even sculpted the "grotesque" meaning people that were maimed or old.

There is so much that can be said about this period, so I'll leave it at that.

On a personal note, my favorite work from this time period is Nike of Samothace from the 2nd Century BCE . If you are looking for a great reference book on this time, I recommend J.J. Pollit's Art in the Hellenistic Age.