On an 18-hour flight from California to Singapore a few years ago, Timothy D. Cook, Apple's chief operating officer, had little time for small talk with a colleague. Glued to his business class seat, Mr. Cook had his nose in spreadsheets, preparing for a thorough review of Apple's Asian operations.
The two landed at 6 a.m., took time to shower and headed into a meeting with Apple's local executives. Twelve hours later, and well past dinnertime, the local executives were ready to call it quits.
"They were absolutely exhausted," said Michael Janes, the Apple executive who accompanied Mr. Cook. "Tim was not. He was ready to jump to the next slide and the next slide after that. He is absolutely relentless."
That relentlessness could be indispensable in the months ahead, because Mr. Cook may be tested as never before. He has been charged with running Apple's day-to-day operations while his boss, Steven P. Jobs, the company's visionary chief executive, is on medical leave.
Mr. Cook has done that twice before, briefly and successfully. Yet if Mr. Jobs's health does not improve, Mr. Cook could be on the job for a long time. And while Apple's succession plans are closely guarded, Mr. Cook is widely believed to be the most likely candidate to permanently replace Mr. Jobs.
In Silicon Valley, Mr. Jobs is also known for relentlessness. Yet on many levels, he and Mr. Cook are opposites. While Mr. Jobs is mercurial and prone to outbursts, Mr. Cook, who was raised in a small town in Alabama, is polite and soft-spoken. He is often described as a "Southern gentleman." While Mr. Jobs obsesses over every last detail of Apple's products, Mr. Cook obsesses over the less glamorous minutiae of Apple's operations.
Their complementary skills have helped Apple pull off the most remarkable turnaround in American business, and made it the world's most valuable technology company. When Mr. Cook is on his own, he will have to compensate for the absence of Mr. Jobs -- and his inventiveness, charisma and uncanny ability to predict the future of technology and anticipate the wishes of consumers.
"He is going to have to look to others to provide the creative vacuum left by Steve," said A. M. Sacconaghi Jr., an analyst with Sanford Bernstein & Company.
Mr. Cook and Apple declined to comment for this article. From his first days at Apple in 1998, Mr. Cook, who is known as intensely private, worked in the shadow of Mr. Jobs and other prominent leaders. Although his job -- making sure Apple could produce, assemble and ship its breakthrough products around the world, and do so profitably -- was not considered sexy, he quickly removed inefficiencies from Apple's supply chain.
"My favorite scenes were meeting suppliers," said a former Apple executive who had traveled with Mr. Cook frequently and asked to remain anonymous because he did not want to upset their relationship. "He is Mr. Spreadsheet. If things weren't right, he would torture the suppliers and demand improvement. At the same time, he had good relationships with them."
Apple was smaller then and largely focused on making PCs. Its operations were a mess.
Apple was still running its own factories in California, Ireland and Singapore. While more profitable and efficient companies like Dell had moved to a just-in-time manufacturing model, Apple still held 90 days of inventory.
Mr. Cook closed Apple's factories and outsourced all manufacturing to a far-flung network of suppliers in Asia. Inventories decreased to 60 days, then to 30 days, then to the just-in-time model. Mr. Cook virtually lived in airplanes, traveling the world to meet with suppliers and browbeat them into meeting his demands.
Analysts and investors say Mr. Cook's efforts on the production end made the difference in turning Apple's fortunes around. And they still are critical to the company's success.
Take the iPad. It took Mr. Jobs's imagination and the expertise of his engineers and designers to create it. But Mr. Cook's operational abilities allowed Apple to parlay a cool product into a business that has already brought in $9.6 billion, as the company built and shipped worldwide nearly 15 million iPads in just nine months to meet customers' seemingly insatiable appetite.
"The ability to ramp up something like that is incredible, and that's largely Tim's doing," Mr. Sacconaghi said. As Mr. Cook delivered results, he earned more respect from Mr. Jobs. More important, because he was focused on areas that Mr. Jobs knew little about, he rarely butted heads with him, former Apple executives said.
Mr. Cook eventually took on oversight of Apple's sales and of its Macintosh division. In 2007, he became chief operating officer, and two years later, he stepped in to run Apple when Mr. Jobs went on medical leave for nearly six months. During that time, he improved the company's financial performance in the middle of an economic downturn.
He also took over for Mr. Jobs briefly in 2004, when the chief executive had surgery for pancreatic cancer.
Mr. Cook received a degree in industrial engineering from Auburn University in 1982. There, too, he had a reputation for his focus. "He was a very quiet, unassuming individual and very, very intense," said Saeed Maghsoodloo, an emeritus professor of industrial and systems engineering at Auburn. "I hardly ever saw him asking questions. He sat quietly and studied."
In 1999, Mr. Cook told Auburn's alumni magazine that he ended up in the computer industry by accident. At a campus event during his senior year, Mr. Cook was nominated to be the "outstanding engineering graduate." After the meeting, a recruiter from I.B.M. who was present persuaded him to join the computer giant.
At I.B.M., Mr. Cook was soon placed on the "high-potential list," for promising young managers, said Ray Mays, a former boss. In a dozen years at I.B.M., Mr. Cook quickly rose through the ranks.
"Tim was the first to work, the last to leave and the smartest guy around the conference table," said Mr. Mays, who was a senior manufacturing manager. While at I.B.M., Mr. Cook completed a master's in business administration at Duke.
Mr. Cook left I.B.M. for Intelligent Electronics, an electronics distributor, and in 1997 took a job at Compaq, the computer maker. Six months later, an executive recruiter suggested that he meet Mr. Jobs. Mr. Cook agreed, even though his friends told him he would be crazy to leave Compaq for Apple, which was considered a basket case.
But Mr. Jobs's charisma and salesmanship proved irresistible.
"Not more than five minutes into my initial interview with Steve, I wanted to throw caution to the wind and join Apple," Mr. Cook said last year during a commencement speech at Auburn.
At 50, Mr. Cook is single and is known for his serious demeanor. Several former Apple colleagues said he rarely socialized with them. His major passion outside of Apple is Auburn football. Former colleagues also described him as a fitness enthusiast who seems to live on energy bars. He is a hiker and a cyclist, who frequently gets up at 5 a.m. to exercise -- and to begin e-mailing his underlings.
"Tim, like Steve, is like a metronome who sets the pace for the rest of Apple," said Mr. Janes, who worked for Mr. Cook for five years and now is chief executive of FanSnap, a ticket comparison shopping Web site. At Apple, Mr. Cook has earned $156.2 million, including salary, bonuses and gains from stock awards, according to Equilar, a company that analyzes executive compensation. In addition, he holds Apple stock valued at about $140 million, Equilar said.
"Without Steve, Apple will be a different company," a former Apple executive said. "But Tim knows what he knows and what he doesn't know, and will trust other guys to do a good job."
The executive added, "He will not be the visionary, but that's O.K. because there are other talented people around him."