In a game that includes Aliens it is sometimes easy to fall into the trap of making every individual conform to the 'stereotype' of its race. Thus, all Wookiees are noble savages with a knack for fixing technology. All Quarren are treacherous bastards. All Mon Calamari are dignified, honorable and dedicated to a cause. All Bothans are sneaky, backstabbing jerks. All Sandpeople are violent barbarians who are easily startled, but return—and in greater numbers, etc..
For a GM, with so much to remember and be in charge of, these stereotypes are an easy kind of short-hand to use to describe a character. Players also seem comfortable with accepting these things—knowing what they can expect from any particular NPC based on race. But while this is a natural inclination, it is also rather silly—especially when you consider how widely different human beings can be in their personalities, behaviors and outlooks.
Much of the RPG (and indeed of the movies, novels and video games) only further reinforce stereotypes. Just look at Zaalbar the Wookiee from Knights of the Old Republic. So he was a bit more 'barbaric' than Chewbacca, but he was still honorable and loyal—and even had a 'life debt' to keep him in the group (just like in Chewie's background). In the adventure module 'Strike Force Shantipole', the Quarren character Salin Glek turns out to be a traitor—which exactly mirrors the RPG's background on the race actually betraying the Calamari to the Empire.
As a GM, it has been fun for me to throw a wrench in this particular trend from time to time, and I would encourage other GMs to do the same. One of the instances I remember most sharply was in the mini-campaign to liberate Mandalore. There, a Bothan (Nef Yahn) was assigned to Rick Oman's team to serve as a clerk/supply officer/overseer for the Republic. Having just read the Timothy Zahn 'Heir to the Empire' trilogy, my players were bristling at the very THOUGHT of one of these Jerks being involved in their endeavors. Before he even arrived on the scene, Nef was demonized—all set up to be the annoying villain of the adventure. And then...he turned out to be quiet, competent and helpful. This... totally threw the players for a loop. Very quickly, opinion on the guy turned around and with just that little twist Nef became a lot more memorable than just a 'generic' NPC would have been.
Other instances of this in my campaign came with an almost 'Femme Fatale' female Wookiee who was a freelance bounty-hunter and adventurer. I'm sad to say I never got to develop her too far, as she was brought in as a 'love interest' for our Wookiee PC, but that character got retired before anything major got developed. She was strong, but lithe and agile—and had a distinctly mercenary attitude that was NOT very wookiee-like.
One of the game modules actually had a nice example of this, too. In 'Death In the Undercity', the characters encounter a Quarren street punk. Though at first seeming to be as shady and untrustworthy as his race was known to be (at least in the RPG), the punk turned out to be an okay guy.
So in short, its easy to take the easy route and make all aliens conform, but in turning that around, its a great tool for a GM to use to keep the players on their toes—and even to confront a bit of in-character 'bigotry' (if you're in to moral dilemmas and the like).