For example, Alexander the Great, leader of the Greeks will be more prone to be aggressive than Caesar of the Romans. Anytime a course of action is not obvious, the AI allows for a bit of surprise as decisions are based on programmed traits, past actions, the current situation of the game. This allows for countless playing styles and is one of the game's defining characteristics. The two best features of Civilization are the depth of its design and its replay value. As a player, you can take countless different strategies in your approach.
Many different variables are present at all stages of the game.
If you choose to become a democracy, you will research well, but you must go to great measures to keep your people happy. Things such as tax rate, luxuries rate, amount of corruption, number of entertainers in a city, military units away from their home city, the existence or nonexistence of city walls or coliseums are just examples of things that can be vital to your success as a civilization. These are just some of the more advanced aspects. You also must decide how and where to build your cities. Do you want to try to found many small cities early in the game or do you want to try and develop two or three large cities.