Each Great Person individual has a different effect, inspired by history. Most effects are strongly linked to a particular era of play, and are not optimal for use outside of that time. The effects fall into two broad categories, Passive Effects and Action Effects.
A Passive Effect happens as soon as the Great Person is spawned on the map. These are mostly found on Great Generals and Great Admirals.
An Action Effect must be activated by the Great Person by using a “charge” at a certain location. The number of charges, the type of location needed, and the effect activated will vary based on the individual. Most individuals only have 1 charge and can only activate at a district matching its type (ex. most Great Scientists have 1 charge and must use it at a Campus). Like other charge-based units, the individual will disappear when all charges are depleted.