At the Battle of Yavin (the final space battle at the end of Episode IV), we see roughly 36 or so rebel fighters lift off to do battle. This is the generally agreed upon number of ships that were in on the attack. During the actual fighting, we see only a relatively small number of these ships engaged- we basically only see Red Squadron and one flight of three fighters from Gold Squadron. When the battle is over, we see only three fighters flying away, along with the Millennium Falcon. These have been established to be Luke, Wedge and one lucky Y-Wing pilot who has since been named 'Keyan Farlander'.
On the Imperial side, we see only Darth Vader, his two wingmen and a handful of other TIE fighters dog-fighting with Red Squadron. Of these, only Darth is shown flying away alive. Obviously, the Death Star could hold (and probably did hold) a heck of a lot more TIE fighters than we ever saw in the movie. The official 'canon' reasoning for them never being launched is that Tarkin wanted to rely on Death Star, to prove that it was invincible. Vader's Squadron of TIE fighters was launched only when there was an inkling that there might be some real danger. I guess this all kind of makes sense. Because otherwise, it would have been 36 X and Y-Wings versus several THOUSAND TIE fighters (in the RPG, the 'official' number of ties carried by the Death Star was 7,200). That... would probably have been a slaughter.
Since the Battle of Yavin is such a pivotal moment in the first movie, it has become fodder for a lot of different stories in the expanded universe. As near as I can tell, the first 'modification' to what we saw in the movies was in the X-Wing computer game (flight combat simulator). I recall one mission to 'clear minefields' or communication satellites or somesuch away from the Death Star- and take out a couple cruisers guarding them. In another, I recall actually flying a B-Wing (or something) on the Trench run. The latter (if I remember correctly), was done just for 'show', and was not to be considered 'canon'. The former, however... is a bit more vague. As fast as the Imperial attack was, I don't see why they'd need to deploy a minefield or communication satellites for that matter, or any cruisers as escorts. As overconfident as the Empire was, I'm pretty certain they just flew the Death Star in.
But I suppose that there is room for this kind of expansion. In fact, in the Star Wars Roleplaying Game, a GM should look for opportunities like this. At the Battle of Yavin, there were at least one and a half squadrons of rebel fighters (18 ships) who we never saw in the movie. Who knows, maybe these guys were attacking 'diversionary' targets at other points on the Death Star. This opens up a lot of room- maybe another elite fighter squadron was released to deal with them. In essence, you could have a whole adventure 'episode' at the Battle of Yavin that doesn't once intrude on what is happening in the movies. Your adventures would just be... a little to the side of the main 'cameras'. Oh, and as afar as only four ships flying away from the Death Star? Well, maybe there were others who flew off in a different direction. Who's to say?
Unfortunately for me, I began my campaign AFTER the Battle of Yavin, so my people never got to participate in it. However, they DID fight in the Battle of Hoth and the Battle of Endor. In any Star Wars campaign where the PCs are rebels, I think it is important that the players ARE involved in these titanic events. I know for me and my people it really helped tie them more closely to the movies- to make them feel as though they were part of the events.
At the battle of Hoth, the group was put in charge of defending one flank of Echo Base. They had a flight of four snowspeeders and a team of infantry- and their own group of AT-ATs to fight. So what if they weren't shoulder to shoulder with Luke and the guys, this way, they got to shine- and not get bound up by what we saw on camera.
The battle of Endor was a lot more tricky to pull off. Some of the party fought in the Space battle (one as a commander of a ship, another as a gunner on the Millennium Falcon.. and.. I think one other gunner, too), others were part of the Endor Strike team. This was tricky at the time since we saw the faces of quite a few of the Endor folks on camera. So basically we filled in the blanks with folks who weren't shown, or who were shown very briefly. For example, when first entering the bunker, we see one of the Rebel troopers clobber an Imperial with the butt of his rifle. We really only see a back shot of this guy, so one of my folks claimed him as 'his character'. Even during the shuttle boarding scene, before the strike team left the mon cal cruisers, you see a hooded figure climb up the ramp of the shuttle. One of my folks- the tusken jedi, oddly enough- claimed this as his character. It wasn't until years later that so many of these 'faceless' roles got names attached to them, which kind of closes up the possibilities for player character insertion if you are a stickler for Canon. Thankfully, I'm not.
So, what is all this rambling about? Hell if I kn- oh! Right. What I'm saying is, big movie events make for AWESOME RPG sessions, as long as you're willing to assume there is more going on than what we see on Camera.