In 2266, DePaul was the ship's navigator during the night shift. At that time, regular night-shift bridge officers included Elizabeth Palmer at communications, Tora Rhada at the helm, Richard Washburn at engineering and Samuel Boma at the science station.
During an assault by a Shechenag vessel, DePaul was first to realize that evasive maneuvers were ineffective. Later, he manned the long-range sensors and gave Spock bearings from a Janus Gate on Tlaoli IV to various Shechenag orbital defense satellites. Using the transporter and DePaul's coordinates, Spock linked the Gate to each satellite and surreptitiously disabled several of them. (TOS - The Janus Gate novel: Past Prologue)
Later in 2266, when an Orion vessel attempted to hide within an asteroid belt, DePaul plotted the positions of all the asteroids by tying into the science station. (TOS - Errand of Vengeance novel: The Edge of the Sword)
In February 2267, DePaul manned the navigator's station beside Hikaru Sulu when the Enterprise encountered a spacial disturbance dubbed Solid Space. He was the ship's highest rated navigator at that time and plotted the quickest route to its center. During their investigation of the phenomenon, the navigation console overloaded and knocked him out. Engineering Ensign Pavel Chekov took over for DePaul, Chekov's first time manning that station. (TOS - New Visions comic: "Mister Chekov")
In 2267, when a distress call was picked up from the Earth civilian vessel SS Harmony from nearly three light-years inside Klingon space, DePaul anticipated Kirk and laid in a course for it. (TOS - Errand of Fury novel: Demands of Honor)
In 2267, DePaul took the helm on at least two occasions. Once during the engagement with the Gorn near Cestus III, and the other during the Enterprise's mission to Eminiar VII. (TOS episodes: "Arena", "A Taste of Armageddon")
In 2268, DePaul was at the helm when more than a dozen two-man fighters attacked the Enterprise from behind an asteroid. DePaul took evasive action but the starship was sluggish compared to the small attack ships. Instead, the ships were lured in closer so that the cloaked IKF Falchion could surprise them. (TOS novel: Dreams of the Raven)
- TOS episode: "Arena"
- TOS episode: "A Taste of Armageddon"
- TOS novel: Mission to Horatius
- TOS novel: Dreams of the Raven
- TOS - The Janus Gate novel: Past Prologue
- TOS - Errand of Vengeance novel: The Edge of the Sword
- TOS - Errand of Fury novel: Demands of Honor
- TOS - New Visions comic: "Mister Chekov"