NEW YORK ( TheStreet) -- Kathy Braddock of braddock + purcell, a real estate consulting firm, says her friendship with co-founder Paul Purcell began 18 years ago when they were simply business acquaintances. After years of working adjacently in the tough world of real estate and then later as co-workers, where they relied on each other for referrals and advice -- even outside of work hours -- they thought it only natural when they came up with an idea to start a business together. Braddock says she liked that Purcell's strong work ethic mirrored hers and appreciated his accessibility and good communication skills. Looking back, Braddock says the similarities in work ethic are a major reason why they are successful business partners today. "He had the same pride in his work and the same ownership of his work, working for a company, as I did working for myself. And he was competitive in the same way," she says. Sharing common interests and career aspirations is one way that friends can be successful at running a business together, but the most important aspect for partners is trust, says Bernhard Schroeder, the director of the Entrepreneurial Management Center at San Diego State University. "I've watched hundreds of people start companies. When people try to marry up on the capability side, if they don't already super-trust each other, when there's trouble the first thing that happens is they start questioning capabilities and the reason they start questioning is because they don't trust the other one," Schroeder says. The pair met in 1994. At the time, Braddock had owned the Intrepid New Yorker, which provided relocation services for Fortune 500 companies. Purcell was working for The Corcoran Group, a large real estate firm in New York, as the head of their relocation group. Over time Purcell moved to Prudential Douglas Elliman where he quickly moved up the corporate ladder, at first heading up the relocation division and later becoming its COO. The two real estate brokers kept in touch and at one point Purcell offered Braddock a job at Douglas Elliman -- his former position -- but she turned him down. A year later he came back to her with a different position as general sales manager, which she finally agreed to in 2000. With a more corporate job, Braddock now had more time to socialize and was able to get to know Paul outside of work. Braddock considered him to be her closest work-related friend. Eventually they began to talk about going into business together. They thought the market needed services to help consumers pick the right realtor. In 2002, the two friends took the jump and opened braddock + purcell. Braddock and Purcell have different approaches to their business: Braddock tends to be more direct and quick to make a decision, whereas Purcell likes to take his time thinking through strategies, but the important thing is it works: They've never had a fight. "We're interchangeable in terms of how we deliver our service," even though our approaches are different, Braddock says. Another large component to making a friendship a successful business partnership is communication. "What should be in place is exactly what the business is going to accomplish," Schroeder says. "The more clarity that you have on every step of the way -- how are we funding the business, how are we going to get customers -- all of those things need to be identified by people, so there are no surprises. It's better that you actually have a really realistic conversation ahead of time in terms of what the business could or couldn't do and then go from there."