Identified as '(Name) the Nobleman' or '(Name) the Noblewoman', they will always wear expensive clothing and lavish homes located in the richest districts, such as Castle Fairfax or the Millfields. They usually also sport posh mannerisms befitting their status, although as is the case with all villagers, this is subject to variation.
In Fable II, nobles are only seen in Bowerstone, specifically around Castle Fairfax. They usually can be seen talking among themselves, telling stories to each other, or listening to lute players. In Fable III, nobles are more common, usually found walking the roads around Bower Lake in Millfields , strolling through the gardens or front courtyard of Bowerstone Castle, and may occasionally be seen perusing wares in Bowerstone Market (Fable III). In the Traitor's Keep DLC, once Professor Faraday has been apprehended and the Hero has helped Huxley in Reconstruction, a large number of nobles will take up residence in the 'Street of the Future' on Clockwork Island.
Being the upper-class citizenry, nobles act according to their status. The nobles of Fable II do not ascribe much worth to the Hero of Bowerstone until they earn more renown. The Hero in Fable III, on the other hand, is initially welcomed by the nobles in the first part of the game, as their status as prince/princess puts them in the higher echelons of society. Although the nobles ignore the Hero once they leave the castle to ready the revolution, they behave like any other villager when the Hero becomes monarch, averting their gaze or bowing appropriately when necessary. Nobles also are among the most difficult prospects for marriage, as they are easily repulsed by certain gifts that they see as 'lower class,' like beer. They also expect a nicer home and upkeep than other villagers.
Although the masquerade-masked people in the Dark Sanctum wear the clothes of nobles and speak like nobles, they are in fact not nobles, at least not as far as the game's classification of nobles is concerned - they will not count as noble kills for powering-up weapons such as Arkwright's Flintlock.