Captain Cornelius Killick appears in Sharpe's Siege, a commander of an American Privateer, Thuella, a New England schooner of sleek, fast lines, built to evade British ships. Thuella had, in fact, become a thorn in the Royal Navy's self esteem. He said she had cost 163,000 'new fangled' dollars, and the investors had made a profit on her returns.
He is described as tall, handsome, and confident. His face tanned by sun and wind, scarred by blades and powder, with sharp features, with dark gold hair. He is on friendly terms with Henri Lassan, commandant of the French fort Teste de Buch. His ship is careened on a beach near the fort awaiting promised wood and copper for repairs. Ducos, who made such promises, however, had no intention of honoring them, disliking tall, confident men. On Lassan's advice, Killick contrived to make his ship look like an abandoned hulk so that it might be ignored by British forces.
He helped Lassan in the defense of the fortress which fell to Sharpe when he and his men took it in a ruse de guerre, claiming they were Americans when asked. After William Frederickson gives the American privateers captured in the action his solemn word they would be treated with the dignity of war prisoners, Captain Bampfylde ignores the promise and Frederickson's objections, calls Killick a common pirate, and having read his Letter of Marque, throws it in the fire and laughs.
Killick swears that not one of his men was British, though he knew about a third actually were, but he would not subject any of his men to a deserters fate. He told Bampfylde, "Piss on you, mister." Bampfylde then has the bound Killick beaten by seven bargemen, Killick lost a tooth as a result. Frederickson demanded his promises to the Americans be kept, and when he was threatened with arrest, went to Sharpe, who cares that Frederickson's word of honour be kept. He halts the abuse.
In a discussion with Sharpe, Killick does everything he can to avoid hanging, including inventing a curse on those who hang sailors in still air, "...Killick lied, but he lied to protect his men, and Sharpe knew that he himself would...lie for his own men." (Sharpe's Siege)
Sharpe asked for their parole, that should they be released, neither Killick nor his men would take up arms against Britain for the duration, Killick gave his word, and Sharpe escorts them from the fort.
When he and Sharpe speak later, he asks release from his promise; though he knew it fair, it irks him. Even so, he feels he owes Sharpe, and informs him that he should burn the barn full of oyster shells. Frederickson then says to do so would result in quicklime. It becomes integral to Sharpe's defense of the fort.
When Killick is later threatened with arrest by Ducos unless he take to sea to bombard the fort, he is torn, until Sharpe secretly approaches him before he sails, and the two make a bargain. The British in the fort surrender to Killick and the United States of America and begin evacuating the wounded to the privateer. When the French try to take the prisoners, Sharpe informs them they surrendered to the Americans. The French then went on the attack.
The Rifles and Marines fought their way to Killick's ship, Killick yelling at Sharpe not to kill Lassan at the last minute, and escape. Killick drops Sharpe and his men down the coast in return for his release from his oath, and invites Sharpe to come drink with him in Marblehead after the war.
Frederickson observes Killick is much like Sharpe, despite being a few years younger, both are hard, good looking men, with the same savage recklessness. He wonders if two such similar men will like or hate one another.
Sharpe likes Killick and easily can picture him as a Rifleman.
In the adaption, the entire naval element, both British and American is eliminated from the story, and the character of Killick never appears in the movies.