We could say that there are at least two roles that Alexander the Great played in trying to create a unified empire.
First, Alexander clearly played the role of conqueror. It is not possible to have a unified empire (or an empire of any sort) unless you first go out and conquer that empire. Alexander seems to have been a great leader of warriors. The fact that he could conquer such a huge empire shows that he must have been very effective in this role.
However, Alexander also played the role of unifier. He did not really want his subjects to think of his empire as something that was exploiting them for the benefit of a faraway emperor. He did not try to destroy the cultures of those he conquered. Instead, he did things like respecting their religions. He allowed Persians to serve as administrators in his new empire. He married Eastern women. By doing these things, Alexander was sending the message that he was ruling for all the people of his empire, not just for his own fellow Macedonians.
In these ways, Alexander played the role both of a conqueror and of a unifier and conciliator.
Alexander, is referred to as the Great, for a reason. He obviously was the driving force of Greece. He helped to make his father's dream of conquering Greece and defeating Persia a reality. He and his father introduced new military methods to the Greeks including the battering ram, catapult, new battle formations and the use of an infantry and calvary. He encouraged his men to fight and they willing fought alongside him. He spread Hellenistic culture throughout the conquered lands, which gained him favor. As oppose to leaders of the past, he did not destroy the conquered lands and force them to submission. Instead, he allowed them to carry-on. They were allowed to continue with their religious beliefs and daily customs. Alexander did leave Greek men behind to help govern, but the people held on to their same way of life. The Hellenistic culture that Alexander helped to create and spread helped to unify the empire as it was a blend of Greek, Persian, Indian, and Egyptian ways of life. In addition, Alexander helped to enrich his empire with the building of the Temple of the Muses (a large museum), the lighthouse of Alexandria, and the Library.
Alexander accomplished many of his goals and left behind a legacy. His desire to securely unify all of his conquered lands failed in the end. After fighting and being away from their families for 10 long years, his men wanted to return home. Alexander wanted to continue fighting and conquering further into India and China, but he returned with his men. Unfortunatley, he fell ill and died before the empire could truly be unified. The empire was eventually divided amongst 3 of his generals. Nonetheless, he is remembered for being great and doing great things for Greece.