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, 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 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 Cook may be tested as never before. He has been charged with running Apple's day-to-day operations while his boss, Steve Jobs, the company's visionary chief executive, is on medical leave.
Cook has done that twice before, briefly and successfully. Yet if Jobs' health does not improve, Cook could be on the job for a long time. And while Apple's succession plans are closely guarded, Cook is widely believed to be the most likely candidate to permanently replace Jobs.
In Silicon Valley, Jobs is also known for relentlessness. Yet on many levels, he and Cook are opposites. While Jobs is mercurial and prone to outbursts, Cook is polite and soft-spoken. While Jobs obsesses over every last detail of Apple's products, 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 Cook is on his own, he will have to compensate for the absence of 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 & Co.
Cook and Apple declined to comment for this article. From his first days at Apple in 1998, Cook, who is known as intensely private, worked in the shadow of 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.
As Cook delivered results, he earned more respect from Jobs. More important, because he was focused on areas that Jobs knew little about, he rarely butted heads with him, former Apple executives said.
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 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.
''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 OK because there are other talented people around him."