In fulfilling online orders, Amazon.com Inc. is all about expediency. The fewer people involved the better.

But when it comes to filling higher-level jobs, the e-commerce giant is in no rush--and it has a gauntlet of people, dubbed 'bar raisers,' who must sign off on would-be hires.

Bar raisers are skilled evaluators who, while holding full-time jobs at the company in a range of departments, play a crucial role in Amazon's hiring process, interviewing job candidates in other parts of the company. With a word, they can veto any candidate, even if their expertise is in an area that has nothing to do with the prospective employee's.

Amazon believes the program, created in the company's infancy and honed by founder and Chief Executive Jeff Bezos, screens out cultural misfits and helps make the e-commerce giant a feared competitor in fields as diverse as logistics, tablet manufacturing and television production.

'There is no company that sticks to its process like Amazon does,' says Valerie Frederickson, whose eponymous Menlo Park, Calif., human-resources consultancy works with Silicon Valley companies including Facebook Inc. and Twitter Inc. 'They don't just hire the best of what they see; they're willing to keep looking and looking for the right talent.'

As Amazon's payroll has swelled to 110,000 employees, however, the program is exacting a toll, current and former employees say.

There are several hundred bar raisers today across the company, according to former employees, though Amazon won't confirm a total. Some employees shun the bar-raiser designation, a voluntaryprogram that comes with no extra pay, even though it reportedly can lead to speedier promotions, because of the time demands. Bar raisers may be asked to assess as many as 10 candidates a week, for between two and three hours each, including paperwork and meetings--all while doing their regular full-time job, be it in finance, marketing or product development.

That has led to a crunch of bar raisers at times, some managers say. Andy Jassy, head of the fast-growing Amazon Web Services cloud-computing unit, said in an interview in November there seemed not to be enough bar raisers to go around and he was looking for more.

Not every Amazon applicant faces a bar raiser. Current and former employees say the company uses a streamlined process for warehouse employees, estimated to be three-fourths of its workforce.

Most others, though, must endure an obstacle course of phone interviews and one-on-one sessions. The interviewers then write evaluations and then meet to discuss the candidate. Inside Amazon, evaluating an applicant typically takes five or six employees at least two hours each.

'We want to be as objective and scientific in our hiring as possible,' said Susan Harker, Amazon's vice president of global talent acquisition, noting the process extends even to C-level executives. 'The point is to optimize our chances of having long-term employees.'

Other tech companies have their own systems for identifying the best and brightest. For a time, Google Inc. asked candidates their I.Q's, and posed brain teasers. Microsoft Corp. calls in senior executives known as 'as-appropriates' in the late stages of considering some applicants. Facebook Inc. asks some job hopefuls tricky coding questions or solutions to business challenges.

The bar raiser is Amazon's distinction. To become a bar raiser, a worker generally must have conducted dozens or hundreds of interviews, and gained a reputation for asking tough questions and identifying candidates who go on to be stars.

Bar raisers typically interview candidates in another part of the company, posing unexpected or challenging questions to gauge an applicant's analytical skills. Current and former bar raisers say the designation is both an honor and a burden.

Sailesh Rachabathuni, who developed software for Kindle devices before leaving Amazon in 2012, says he once vetoed a candidate for a programming job because the candidate didn't know much about a specific programming language, a detail others missed.

'It's an enormous time commitment,' Mr. Rachabathuni says. 'I had to limit myself to six interviews a week.'

One of Mr. Rachabathuni's former colleagues in Lab126, Amazon's secretive Silicon Valley hardware laboratory, says he conducted more than 700 interviews over eight years at Amazon. But this ex-employee declined to become a bar raiser for fear of devoting more time to hiring.

In cultivating the program, Mr. Bezos wanted to create a consistent corporate culture. Amazon executives say the approach reduces hiring mistakes by forcing several people to sign off on a candidate. The program is 'something the broader team is very proud of,' Mr. Bezos said in an interview last year.

John Vlastelica, an early Amazon human-resources employee who helped design the program, said the tough review process was meant to weed out job hopefuls who aren't adaptable and may be skilled at only one task.

'You want someone who can adapt to new roles in the company, not just someone who can fill the role that's vacant,' said Mr. Vlastelica, who now runs HR consultancy Recruiting Toolbox and counted Amazon among his former clients. 'It can be an expensive process because it takes longer, but think of how expensive it is to hire the wrong person.'

The burden is likely to grow as Amazon extends its torrid hiring pace, needed to staff its ambitious efforts to expand same-day delivery, and to build Kindle tablets, as well as smartphones and set-top boxes. In the 12 months ended Sept. 30, Amazon added close to 30,000 employees, roughly as many as eBay Inc.'s total payroll. The retail giant's workforce has more than tripled in the past three years.

Google contracted to 46,421 employees as of the end of September 2013, from 53,546 a year earlier, due in part to cuts at its Motorola Mobility division. Apple Inc. grew by 10% to 80,300 in the fiscal year ended Sept. 28, and Microsoft Corp. increased its staff by 5% to 99,000 in the year ended June 30.