As any other form of mannerism, Hellenism was driven by the effect more than by the canon. Rules were abandoned or reconsidered for the sake of more impressive art solution.
Development from archaic art to Hellenism in Greek art was a development from the concepts of reason and harmony to the celebration of emotionally driven exterior effect. Classic art in Ancient Greece is a pinnacle of absolute artistic perfection. It is a moment of discovery of ideal of bodily beauty. This ideal was formed as a rational canon and was pretty much impersonal. Emotions were not a part of this ideal and, therefore, they were left out of the field of interest of classical art. All Greek classical art is pretty much emotionless.
In Greek Art:
"The principles they most revered were moderation, balance, self control, all summarized by the words attributed to Solon carved in the stones of their holiest shrine, Delphi: 'Nothing in Excess.' The Greeks embraced this golden mean because they were a passionate, individualistic people, quick to vent their emotions, and they realized how difficult moderation is to achieve. Every Greek considered himself a battlefield where Apollo's reason and Dionysos' passions struggled for control."
Emotions in Hellenism became so theatrical that they do not look real. Look at Laocoon as an example.
‘Laocoön was the Trojan priest who warned Troy not to accept the Horse. The statue portrays Laocoön and his two young sons being attacked by snakes. The work is very powerful, but it does have its faults. For example, Laocoön's face shows an old man, but his body is that of a 25-year-old!”
In the sense, the whole idea of Hellenism was to replace cold reasoning of classicism with emotionally driven experiments. Formal achievements of classicism were preserved. But Hellenism was no longer limited by formal canon. In other words , same forms that were created during classicism were filled with different content:
“While the Classical Greek concepts were not entirely abandoned, the artist of the Hellenistic era expanded his formal horizons with dramatic posing, sweeping lines, and high contrast of light, shadow and emotions. The conventions and rules of the classical period gave way to the experimentation and a sense of freedom that allowed the artist to explore his subjects from different unique points of view.”
