Over the last two and a half years, I traveled across the country in search of life-changing teachers and mentors from all different walks. I met race-car drivers, Indian potters, ballet dancers, rappers, research scientists, law professors, Montessori teachers, aerobatic pilots, master carpenters, and many others. The book that emerged from those travels is called Guiding Lights. It tells the stories of several of these remarkable people and the ways they transform their apprentices.

First Sgt. Peter Hall looks like he walked straight off a Marine recruiting poster. He's tall, fierce, and imposing, with a deep voice that calls forth both his native Jamaica and the Bronx of his adolescence. A 19-year veteran of the Marine Corps, Hall was the drillmaster of Marine Corps Officer Candidates School when I first met him three years ago. He'd been a drill instructor for six years. I caught up with him again at Camp Pendleton, Calif., where many Marines, including Hall and his unit, had recently returned from combat in Iraq. They will head back there soon. Our task this morning, with the sound of target practice in the distance and helicopters overhead, was simple and daunting: First Sgt. Hall was going to teach me to drill a squad of 12 Marines.

     Close-order drill is the precise marching and movement of a unit in formation. To some, drill might seem like mere ceremonial flourish, a parade-day distraction from the real stuff of war fighting. It is no distraction. Drill teaches discipline, teamwork, attention to detail, and something else vital in this environment: total obedience to orders.

     In the summers after my sophomore and junior years in college, I went to Marine Officer Candidates School in Quantico, Va. I went out of patriotism. I went to discover what it meant to command, and to glimpse more of my true character. The 12 weeks were brutal and glorious. I chose ultimately not to take the second lieutenant's commission. But my time with the Marine Corps raised a question I've often returned to: How much of being a good leader is teachable? Can command presence be instilled?

     First Sgt. Hall brings me to a Camp Pendleton parking lot to train a squad of Marines in close-order drill. The dozen Marines are all active-duty enlisted men from Hall's unit. Hall first demonstrates the commands he wants me to give: FALL IN, RIGHT FACE, LEFT FACE, ATTENTION, PARADE REST. His voice is huge, booming, unambiguously clear. He blasts at operatic volume, without any obvious effort.

     I give it a try. My voice is like a pocket version of his. Plenty of spirit, but pitifully little volume. Poor articulation and pacing. This I can hear and feel. But then Hall tells me I need to raise the pitch of my voice. This surprises me. If anything, I'd been trying to deepen my voice, to project what I thought was a more masculine sound. His suggestion was purely practical, though: A commander has to be heard, period. If I am too low in register, there will be confusion in the ranks about what I said. A higher tone carries more effectively and will be carried out more effectively.

     I was supposed to know this already, this idea of what voice to project. When I'd been an officer candidate 16 years earlier, my platoon commander had pulled me aside one day and counseled: "Liu, when you give orders, you drop your voice down. You're trying to sound like a commander. Don't do that. Just command."

     But that's the thing: The expectations of others can make you forget the register of your own voice. The young Marines I am facing now aren't in their most intimidating garb: They're wearing running shoes and sweatshirts. I'm still incredibly nervous. And now it's time for me to march these Marines. Hall takes them for one loop, his practiced call echoing across the lot.

     When he turns the squad over to me, I literally get off on the wrong foot: I call "left face" when "right face" was appropriate. Out squirts a tiny nervous smile, what would have been, in a "real" situation, an instant obliterator of my own authority. Hall demonstrates now how to back out of the mistake—by calling "right face" twice in a row—and more important, how to keep my cool while doing so.

     "For-ward … HARCH!" From then on, it's a disorienting, exciting, bumbling, beautiful process. I am in command. It matters not how completely artificial the situation is, how especially respectful the Marines are being toward me, and how relatively gentle First Sgt. Hall is in his real-time critiques. I am stressed out. My eyes are fixed on the marching feet of the squad, and Hall senses it immediately. He comes up next to me, telling me urgently to stop following their feet and to simply call a calm cadence. "Think about you marching, not them. If you follow the cadence you set, so will the squad."

     I'm doing that, and appreciating what good advice this is, when I suddenly realize I've forgotten to call "Column Left." Without this order, the Marines dutifully march forward, heading straight for a wall. You can hear the nervous embarrassment in my voice, and the quick stifling of it as I try to regain control. For half an instant, I am no longer calling, "A lef ri lef" in that sonorous Marine way. For half an instant I am speaking, hesitating, like an unsure civilian.

     But I regroup. Just in time, I turn the column to the left. They recover, and so do I, and the rest of the drill session goes smoothly. I recover because First Sgt. Hall has been by my side the whole time, guiding me sotto voce, telling me it's not too late. What made him one of the best drill instructors at OCS was not simply that he could, with the flip of a switch, play the "stress-monster" of movie lore—stomping and screaming, mocking the poor candidates. What has made him such a skillful teacher is that he believes in the possibility of drawing untapped, unseen potential out of his pupils. He let me make the mistake, then he quietly but firmly told me that I would fix it. And I did.

     On this day at Camp Pendleton, I learned not just how to drill a squad of Marines. I learned how to carry myself. And in so doing, I was completely emulating the model closest to me: Peter Hall. Indeed, as I listened later to the recording of our encounter I realized that every time he interjected with a command or a demonstration, I then had unconsciously matched my timbre and tone to his.

     I was mortified that I was so completely malleable and imitative. But then I remembered something Hall told me: It took him nine years of being a Marine before he found his own voice for drill. Nine years of careful watching and listening, picking up bits of one man's style and fragments of another's, cobbling the sounds and distinctive marks of diction into his own composite sing-song call.

     That was one of the most humbling things I learned on this day—that the increments, the drill and repetition, take so very long to accrete into a core sense of what it means to lead authentically. But it was also one of the most exciting things I learned—that everyone's voice is a composite, authentic and synthetic at the same time.