LIKE CLARK KENT, who ducked into a phone booth to transform into Superman, Stephane Lopez is a quick-change artist.

Last summer, Mr. Lopez was covertly interviewing for another job while still working at a New York investment bank. The investment firm had adopted a casual dress code, which usually meant khakis and a polo shirt. But a suit and a tie were more appropriate for his job interviews.

Choosing the right clothes for an interview has always been daunting, but it's even more so today, when so many rules have been broken -- then reinstated. And then there's the issue of trying to fit into your regular workday.

Dressing well for interviews without arousing the suspicion of his co-workers became a headache for Mr. Lopez, now 27 years old. "If everyone is very dressed-down and you are not, someone will raise their eyebrows," he says. "It was stressful. I always had to plan ahead."

Online career sites are filled with postings about interview dress strategies when candidates are currently employed. Postings on Vault.com, for instance, suggest leaving interview-ready clothes in the car or in health-club lockers near the office, where workers can quickly change.

MR. LOPEZ DEVISED his own creative gambits. He scheduled one interview with a consulting firm at 7 a.m. That way, he could wear a suit to the interview, then go home and change again to be casual for the investment bank.

The day of an afternoon interview, he "medium-dressed." To work, he wore a suit jacket and pants, and a polo shirt. But carefully packed inside his bag were a dress shirt and tie. Before the interview, Mr. Lopez allotted an extra 20 minutes to stop in a public bathroom and change into the dressier attire.

While a suit and tie might be appropriate for New York banking and consulting interviews -- which have grown even more formal after the dot-com crash -- jobs in other industries or regions can have more lax interview dress codes.

"You need to dress the part of where you want to be," says Paul Capelli, a former public relations executive at Amazon.com. The Seattle online retailer had an extremely casual environment, says Mr. Capelli, now the vice president of public relations at CNBC. (Dow Jones & Co., publisher of The Wall Street Journal and WSJ.com, is co-owner with General Electric of the CNBC television operations in Asia and Europe, and provides news content to CNBC.)

Mr. Capelli says when he and his co-workers conducted interviews at Amazon, they would often "snicker" at the interviewees who were dressed in fancy suits and ties, he says.

"When you're trying to make an impression in an interview, you want to prove that you are going to fit within the organization," he says. "If the culture is jeans and you come in a three-piece suit, it is not working."

The over-dressed candidates, Mr. Capelli reasons, didn't properly investigate the company's culture. "It sends a little flag: Is this someone who has done homework and is going to be the kind of person who has the savvy to be part of the team?"

Sometimes he'd try to help the candidates before they faced their next questioners. "Take off that tie; take off that jacket," he'd tell them.

Mr. Capelli has developed his own rule for dressing right for a job interview: Find out how the company dresses and notch it up one step.

"If the dress is jeans and a T-shirt, wear slacks and an open collar shirt," he explains. "If it's slacks and an open collar shirt, throw on a sport coat. If it's a sport coat, throw on a suit. At least match it and go one step up, but don't go three steps down."

CAREER COUNSELORS generally advise interviewers to err on the side of formality. Jeans and sneakers are a don't for most jobs. Interviewers want to know that a candidate takes the interview and the job seriously.

"However they present themselves to me is how they will present themselves to the firm's clients," says Maury Hanigan, CEO of Hanigan Consulting Group, a New York human-resources strategy firm. "If they are too casual or too flip or too laid back for an initial meeting that can come across as indifferent or arrogant."

Women generally have it easier than men, adds Ms. Hanigan. "Women can straddle dress standards more easily," she says. "With men it's much harder. Either they are in a suit or not."

There is a fine line between looking respectful and looking ridiculous, career counselors say. "Nobody wears three-piece suits anymore," says Kate Wendleton, president of the Five O'Clock Club, a national career-counseling organization based in New York. "And women can wear too much jewelry or too much of a hairstyle. You can look overly dressed for an interview."

If a candidate has no idea what the company's dress is like, it's OK to ask the recruiter or hiring manager before the interview, counselors add.

Mr. Lopez did just that. He called Ms. Hanigan, who was interviewing him for a job as a research associate, to explain that he worked in a company with a casual dress code. Would it be all right if he came to the interview casual, he asked. "He didn't want to red-flag his employer," recalls Ms. Hanigan.

Mr. Lopez got the job.