The Star Trek Writers Guide describes him thusly:
''Played by William Shatner , Kirk is about thirty-four, an Academy graduate, rank of Starship Captain. A short hand sketch of him might be "A space-age Captain Horatio Hornblower", constantly on trial with himself, a strong, complex personality.
With the Starship out of communication with Earth and Starfleet bases for long periods of time, a Starship captain has unusually broad powers over both the lives and welfare of his crew, as well as over Earth people and activities encountered during these voyages. He also has broad power as an Earth Ambassador to alien societies in his galaxy sector or on new worlds he may discover. Kirk feels these responsibilities strongly and is fully capable of letting the worry and frustration lead him into error.
He is also capable of fatigue and inclined to push himself beyond human limits then condemn himself because he is not superhuman. The crew respects him, some almost to the point of adoration. At the same time, no senior officer aboard is fearful of using his own intelligence in questioning Kirk's orders and can themselves be strongly articulate up to the point where Kirk signifies his decision has been made.
Important -- Although Kirk will often solicit information and estimates from Spock, never does the first officer act as Kirk's "brain". Our Captain is a veteran of hundreds of planet landings and space emergencies. He has a broad and highly mature perspective on command, fellow crewmen, and even on alien life customs, however strange or repugnant they seem when measured against Earth standards.
On the other hand, don't play Kirk like the captain of an 1812 frigate in which nothing or no one moves without his command. Speck, McCoy, Scotty, Sulu and Uhura are a trained team and are well able to anticipate information and actions Kirk needs.
Aboard ship, Captain Kirk has only a few opportunities for anything approaching friendship. One exception is Mister Spock, a strange friendship based upon logic, high mutual respect and Spock's strong Vulcan loyalty to a commander. Another is with ship's surgeon, Dr. McCoy, who has a legitimate professional need to constantly be aware of the state of the Captain's mind and emotions. But on a "shore leave", away from the confines of self-imposed discipline, Jim Kirk is likely to play pretty hard, almost compulsively so. It is not impossible he will let this drag him at one time or another into an unwise romantic liaison which he will have great difficulty disentangling. He is, in short, a strong man forced by the requirements of his ship and career into the often lonely role of command, even lonelier because Starship command is the most difficult and demanding task of his century.