How does Shakespeare make Caesar such a memorable character despite appearing in less than half of the play?

Julius Caesar's assassination drives the play, so Shakespeare makes him a memorable figure by necessity. The Roman population are enamored with his military victories and charisma. Before Caesar even appears, the audience is given conflicting views of the man as both imperious tyrant-to-be and as a glorious demigod. When he appears at last, Caesar is neither. He is a superstitious, vain man, though his capacity for evil is never laid out clearly.
In his appearances, Caesar strikes the audience as pompous, speaking in third person and displaying great arrogance, even claiming moments before his assassination that he is as "constant as the Northern Star,/ Of whose true fixed and resting quality/ there is no fellow in the firmament" (Act Three, Scene One, verses 60-62). His vanity and belief in his own press so to speak make him seem a candidate for tyranny. However, Shakespeare does not make Caesar out to be a mere tyrant. He is weak and superstitious, but whether or not he would have been a bad ruler is never spelled out. This ambiguity makes him interesting.
Caesar is further made memorable due to his impact on the other characters, an impact which lasts well beyond his death. The tragedy of the play comes from the reaction to the assassination, the strife it reaps in Rome. Caesar's ghost appearing to Brutus in the last act is a representation of how Brutus is still haunted by his former friend. Brutus and the conspirators may have killed Caesar the man, but as a symbol, Caesar remains very much alive.
http://www.shakespeare-online.com/plays/juliuscaesar/juliuscaesarcharacters.html