Scottrade CEO and founder Rodger Riney expects his employees to have fun at work. But on April Fools’ Day — during the height of the flash-mob craze — even he couldn’t have guessed that his staff would surprise him with a dance routine to Taio Cruz’s “Dynamite.”

In a video posted on YouTube, the wide-eyed look on Riney’s face serves as a reward for the month of rehearsals put in by a troupe of a few dozen financial services professionals.

True, flash mobs don’t happen every day at Scottrade. Celebrations and camaraderie, however — including free birthday lunches, ice cream socials, cook-offs, and a recent carnival for employees and their families — are routine at a company that has appeared on Fortune’s 100 Best Companies list for the past six years.

When Riney founded Scottrade in Scottsdale in 1980, he embedded a “friendliness requirement” in the firm’s hiring. As Riney told Fortune, he wanted “to hire nice people who are honest … and that I want to see every day.”

That rule held as the firm grew. To expand its reach in recent years, the company asked employees to suggest new locations. A small number of associates took advantage of the offer and relocated, helping to open offices in 48 states. Today Scottrade counts more than 3,500 employees and 505 branches in its network.

Riney also believes that life is too short to work for a paycheck alone (though all employees, who don’t work on commission, receive semiannual profit-sharing bonuses). “Your job should be enjoyable and fun,” he says.

In turn, employees hail his approachability. Wendelynn Rhodes, who joined Scottrade in 1998 and now heads the brokerage’s Eastern division, felt “starstruck” when the CEO called her on her five-year anniversary with the firm. At the time she worked as a branch manager in Greensboro, N.C. “I couldn’t believe it,” she says.

That personal touch is evident even in hard times. Last year, in an effort to boost profitability and bonuses, Scottrade cut $33 million in costs — but chose to reduce the travel budget rather than trim perks.

Until earlier this year, the company had never had a layoff, but lackluster trading volume prompted Riney to restructure the company. “We dealt with that in the most delicate way that we could,” he explained. In the end, only 13 employees were let go, and 25 others were relocated within the organization.

Matt Wilson, who previously worked at competitors TD Ameritrade and Charles Schwab, was hired to lead Scottrade’s brokerage unit. One day last December he heard singing down the hallway. When he went to investigate and found a band of employees caroling outside Riney’s office, he knew he was in a different kind of place.

This story is from the November 18, 2013 issue of Fortune.