One of the major themes/ideas explored in the Dead Poets Society is that of freedom versus conformity.
The theme of conformity is introduced in the very opening scenes of the film. Close-ups of the boys chanting in unison, all identically clad in their starched uniforms at a ceremony at Welton Academy, we see that they are conforming to the authority of the school.
It is the new teacher Mr Keating who, through his unorthodox methods, encourages the boys to challenge this authority, and break free from the traditional, conservative ways of thinking that have been drilled into them at Welton Academy. He wants them to understand that there is more to life than obeying the orders of others, and in this way the film deals with Weir's common theme - also explored in Witness and Gallipoli - the quest for personal freedom, and the oppressing effects of society's institutions.
He inspires them to "Maintain thoughts and beliefs in the face of conformity." Keating wants them to become "free-thinkers", but he is in a way contradicting himself as he forces his own beliefs and philosophy onto the impressionable students rather than letting them think for themselves.
This theme of conformity is paralleled on a personal level in Neil's relationship with his father. Mr. Perry wants the best possible future for his son, and therefore has almost impossible expectations of him. Neil, on the other hand, while always obedient to his father's wishes, wants to know more about himself. Acting was something that Neil discovered he was not only good at and enjoyed, but was also in a sense an escape from his present reality as it allowed him to pretend to be someone else for a while.
Mr Keating's "carpe diem" ("seize the day") attitude inspired Neil to disobey...