"Watch out for people who have a situational value system, who can turn the charm on and off depending on the status of the person they are interacting with," Swanson writes. "Be especially wary of those who are rude to people perceived to be in subordinate roles."

The Waiter Rule also applies to the way people treat hotel maids, mailroom clerks, bellmen and security guards. Au Bon Pain co-founder Ron Shaich, now CEO of Panera Bread, says he was interviewing a candidate for general counsel in St. Louis. She was "sweet" to Shaich but turned "amazingly rude" to someone cleaning the tables, Shaich says. She didn't get the job.

Shaich says any time candidates are being considered for executive positions at Panera Bread, he asks his assistant, Laura Parisi, how they treated her, because some applicants are "pushy, self-absorbed and rude" to her before she transfers the call to him...

Such behavior is an accurate predictor of character because it isn't easily learned or unlearned but rather speaks to how people were raised, says Siki Giunta, CEO of U.S. technology company Managed Objects, a native of Rome who once worked as a London bartender.

More recently, she had a boss who would not speak directly to the waiter but would tell his assistant what he wanted to eat, and the assistant would tell the waiter in a comical three-way display of pomposity. What did Giunta learn about his character? "That he was demanding and could not function well without a lot of hand-holding from his support system," she said.