There were a number of considerations when selecting the area to invade: air cover, naval protection, strategic possibilities and logistics.

Firstly the whole invasion area would be need to be covered by constant air patrols; the RAF's fighter bases were mostly in the south-east of England and their primary fighter (the Spitfire) had a relatively short range, setting a western limit of Cherbourg and an eastern limit of Flushing to start with.

From a naval perspective it was highly desirable to operate in the English Channel between Cherbourg and Calais; this area minimised the risk from German mines, submarines and surface units. Operations east of Calais into the North Sea would not have been impossible, but would have required a much heavier minesweeping and defensive naval effort.

Strategically it was desirable for the target area to be lightly defended, but consideration had to be given to launching offensive operations once the lodgement area was secure, including capturing or establishing airfields.

Logistically the invasion force would need to be kept supplied over beaches at first - the 1942 Dieppe raid had shown the difficulty of capturing a port, and ports within invasion range had been even more heavily fortified in the intervening years. The lodgement area would need to contain ports, though, to increase the flow of supplies as the Allied forced built up.

Studying the beaches from Cherbourg to Flushing, the Dutch beaches were considered unsuitable due to limited road exits, an insufficient tide range for "drying out" (grounding a ship at high tide and unloading onto dry sand as the tide goes out) and exposure to prevailing winds. The Belgian beaches were also exposed to prevailing winds and strong currents, and backed by extensive sand dunes. Combined with the increased difficulties in naval operations east of Calais, the logical choices were therefore either the Pas de Calais or the Cotentin-Caen area. The former had the advantage of being the closest point to the UK, but as a result was the most heavily defended by the Germans, hence the selection of the latter.

The attitude of citizens was not a major consideration; the possibility of a massed national uprising was discussed but considered unlikely. The Special Operations Executive (SOE) co-ordinated with local resistance fighters to support the invasion through sabotage and disruption, but these were a 'bonus' rather than a fundamental part of the plan.

The US Naval Administrative History of World War II Volume V: The Invasion of Normandy and The United States Army in World War II, European Theater of Operations: Cross-Channel Attack both cover the considerations and planning for Overlord in some detail.