Therin of Andor wrote:
a combination of a really good audition on the part of Tim Russ and the desire to cast someone young enough to handle a tough shooting schedule with its share of action scenes.
Actually, the reason why Mark Lenard, Suzie Plakson, Jeffrey Combs, Tim Russ, Marc Alaimo, Tom Morga, Armin Shimmerman and a few others got so much repeat work in "Star Trek":
hugely talented, very cooperative, friendly demeanor, and a proven record for absolutely no loud complaints about working under hot latex appliances.
Not to mention that when they are in make-up for aliens it is hard to tell their characters apart. I didn't recall picking up on J.G. Hertzler being both the Vulcan Captain in "Emissary" and Chancellor Martok until he made that comment on the DVD bonuses about how his last DS9 appearance was as a random, human extra in the end scenes of the finale.