I know I've mentioned it before somewhere, but the issue of Feature Characters (FCs) in a campaign is something that seems to demand closer inspection. For the uninitiated, Feature Characters are Non-Player Characters who are considered great heroes in a game setting. In Star Wars, these would be Luke, Leia, Han, Chewbacca, etc.. In any game system, I believe that the player characters should be the focus of the story. There is some basis, therefore, in the argument that including Feature Characters takes the spotlight off of the Player Characters (PCs). Used in the wrong way, FCs can do exactly this. But used correctly, they can have the opposite effect of having the players begin to feel that they are operating 'on the same plane' with these epic heroes—and that is something that I ultimately encourage in my games.
What I consider the 'right way' of using FCs consists of several maxims which I will explore below:
1) Introduce Feature Characters Subtly
Try not to make a big deal out of a FC's appearance. Try not to do stuff like: "This week's adventure features Luke Skywalker!" PCs shouldn't really know about it until its all the sudden "Oh, hey Luke." They should fall into the 'surprise guest star' role. A great example of this (and of many of these maxims) is found in the Minos Cluster campaign (presented in Galaxy Guide 6: Tramp Freighters). In one series of adventures, the PCs are told they will be meeting and escorting an Alliance diplomat around the cluster. They will likely be surprised to discover this 'diplomat' is none-other than Princess Leia.
2) Use Feature Characters Sparingly
It's okay to have a FC team up with the PCs for a particular mission. But familiarity breeds contempt—i.e. the PCs will likely not continue to be impressed if Luke is teaming up with them every other mission. Also, this is the biggest danger-point for FCs overshadowing the PCs. Han Solo is bogus. And if he's in every mission, then its going to leave the PCs with very little to do. Again, the Minos Cluster campaign is a good example of this: Leia teams up with the characters for this one mission, then goes on her way. It provides a good basis for interaction in the future (i.e. she knows the characters by name) but nobody overstays their welcome.
3) Establish Personal Relationships
Try to provide 'moments' of interaction between FCs and PCs—these should be social, conversational, humorous or serious, just anything that doesn't particularly relate to the mission at hand. It helps to lend depth to the FC in question as well as providing a personal link between PC and FC other than "we just happen to be on the same mission." I never did this enough in my original Vermillion campaign. And looking back on it, I regret that and hope to make amends in the future. In my campaign with Adren, however, I used this extensively. She got a chance to hang out with various Feature Characters, and in the end, she became great friends with several (and enemies with a few, too).
4) Let the Player Characters Take the Lead
Those (hopefully rare and memorable) times when PCs and FCs work together, make sure you give the PCs their own moments to shine. In no way should they be forced to take the back seat while Luke displays his lightsaber prowess or Han takes out all the TIE fighters single-handedly. Again, going back to the Minos campaign, Princess Leia is doing diplomatic work while the players are responsible for her security. This is a pretty important task, considering who Leia is. Depending how the adventure goes, the PCs even get a chance to join the 'I rescued Princess Leia' club. Accomplishing something like this—bailing out a powerful FC—should make the PCs proud.