Time allows you to get to know someone. Time reveals someone's subtle characteristics. Not just negatives, but positives too. I've hired people who I at first believed weren't really the right fit, but over time, I discovered the real person behind the first impression. You must be willing to take your time if you want a staff whose sum is greater than its parts.
Members of successful teams differ in complementary ways. If you've ever worked for someone who only hires people like themselves, you know why this is true. Diverse thinking requires diverse thinkers. Differing perspectives will keep your agency's work from seeming similar, regardless of which client it is representing.
One of the reasons our agency tries to be as collaborative across our disciplines as possible is to never be accused of having a "style." For example, when one of our account planners is developing a strategy, he or she will have development sessions with team members from the media and creative departments. The same is true of each department as they develop their segment of a project. This requires a great deal of trust between our departments because they're showing their thinking before it's fully baked. They also are allowing others to play in their sandbox. This isn't to suggest that account planners create the ad or art directors determine a media plan, it's simply allowing the entire team to form an approach together. This kind of environment requires a rare type of employee.
Finding people who aren't carrying the baggage of bad inner-agency relationships, have an insatiable appetite to grow and realize that building something special requires time; resolve and self-sacrifice should a pre-requisite to talking to someone about joining your agency. To discover that much about a person takes time, but getting rid of a mistake is a greater burden.
Years ago, I had a client who had a simple philosophy about employees: "Let the eagles soar and shoot the turkeys." At the time, I remember thinking that sounded a bit harsh. As the years went by I realized just how integral to success that philosophy is. If you want a successful company, if you want to be a leader in this industry, let's face it, if you want to survive, it's the way things have to be. The unpleasant thought of shooting a turkey should be adequate motivation to avoid hiring one in the first place. From a distance, eagles and turkeys are just birds. Get close enough to see the difference before you're left in the position of having to get out your gun.