So, in order:
GreenTongue:
Would a freely accessible movie be too much?
Films would probably be better than books, not least because they tend to be quicker and you can multi-task while doing them - there's also statistically a higher chance that more of an audience will have seen the film or something similar.
GreenTongue:
To establish a common framework for the setting, is not a book or movie useful?
I'm not disagreeing that it can be very helpful, but you've also got to consider what's practical. In an ideal world, I'd be in a situation where every game I set would potentially have multiple media reference sources which would mean everyone's on the same page as me, but I think you're always facing an uphill battle given the demands on everyone's time.
I'd also cite Donsr's point above about different people getting different things from a film or a book, and there is a further risk that, if you're just looking for a general setting point, that people will become too wedded to the book and potentially not accept/like any changes you make to the canon.
GreenTongue:
Is it unnecessary, do you think, to have an established setting in common?
Have you found that developing during play is more than enough?
It depends very much on the game, but in short, doing the prep work accordingly (both in scene setting and in RTJs) and during your play should be enough.
Established settings can also be a double edged sword - building on Donsr's point, if I ask four different players to make me a Star Wars character, you could easily end up with:
- One comedy Gungan character called Mor Cha Bangs, played very tongue in cheek who blunders around in search of food.
- One sadistic, unpleasant Sith Lord who revels in the pain and suffering of others, played as an edgy experiment on how far you can push it.
- One deep character study of loss in the form of a surviving Order 66 padawan, which is mainly meditative and focused on character development.
- One min-maxed Mandalorian with extra arms and immunity to lightsabers, who just wants to shoot things and make funny quips.
All of them are potentially valid within the overarching Star Wars setting, but you'd never want to put them all in a group together, because they all want different things from the game and wouldn't get along as a group.
That's where you'd need to be very specific as a GM as to what you want to draw out from a setting and from a game.
GreenTongue:
The idea, in my mind, is to have that common feel for where the play takes place.
Yes, you can certainly learn as you go but, some level of expectations of how things are, I find helpful.
I absolutely agree on wanting to find a common theme, but you need to be efficient about how you go about that process, both in terms of taking up potential players' time and in making sure that the examples you give actually provide players with the setting you need.
So for the original example, I'd probably either write some flavour text myself based on the original book (perhaps even picking a very evocative paragraph), or I'd find something similar and link it across (perhaps a clip from Jason and the Argonauts, or Assassin's Creed) with some instructions about what I was specifically looking for.
You may also find that something completely different is a better evocation of feel than asking someone to read a whole book - given the trading feel, you could perhaps say something like 'Firely, but set in Ancient Greece' if that's pertinent to the feel you want.