Leon Carroll Jr. Is a retired NCIS Agent and a technical advisor for the drama series NCIS. Originally a member of the United States Marine Corps, Carroll later joined the real-life NIS/NCIS agency and worked there for two decades before retiring. After retiring, he returned to help in the aftermath of 9/11 before retiring again. Carroll later joined NCIS and now serves an adviser, advising the NCIS writers on what a real-life NCIS Special Agent would do. NCIS Director Leon Vance, who is played by American actor Rocky Carroll (no relation to Leon Carroll), is partly named after him. In addition, Vance's habit of chewing on a toothpick is inspired by Leon Carroll's real-life habit of chewing on a toothpick.