Note: I'm asking this question due to an ongoing discussion on the message boards of The Force.Net concerning what could potentially happen to the existing media of the Star Wars 'Expanded Universe' - the majority of which takes place in the years after Return of the Jedi - now that we know that the Sequel Trilogy is coming and will be covering that same time period, and would like to be able to offer some specifics concerning how writing tie-in media generally works.
If you're looking for some universal practice regarding how canon and tie-ins are related, there isn't one. It depends on the individual franchise and the decisions of the people running it. A lot of tie-in franchises don't worry too much about continuity, or even about accuracy to the source in some cases I've seen. Star Trek
has generally maintained a policy that the books and comics have no impact on the canon, and though there have been occasional periods of continuity among different books or comics, in general there's been no effort to maintain a uniform tie-in continuity. Pocket novels currently have an interconnected continuity, but there are still novels that are separate from it, and the comics and Star Trek Online
game follow their own distinct courses. As for Star Wars
, the licensed tie-ins have long been treated as a continuous universe and have sometimes influenced canonical material, but as is always the case, the creators of canon have the option to rewrite the universe (including reinterpreting past canon) as needed to serve a current story. (The fan perception that canon is unalterable gospel is entirely wrong. It's all just made up to begin with, so it's easy enough to pretend some earlier part of it happened differently than was originally pretended.) Then there are those occasional instances where the creator of the canon personally supervises the creation of canonical tie-ins, as J. Michael Straczynski did with the majority of the Babylon 5
novels and all the comics, as Rockne S. O'Bannon did with the Farscape
comics scripted by Keith R.A. DeCandido and David Mack, and as Joss Whedon has done with the comics tie-ins to his shows (although the canonical Buffy/Angel
comics were preceded by years of non-canonical comics and novels).
Basically, the only universal rule of tie-in writing is that we're hired to work for the owners of the franchise and our job is to follow their lead and their instructions. Generally that means we have to stay consistent with the canon as it currently stands, but they don't have to stay consistent with what we do. If they call our books canonical, as Lucasfilm has done with its tie-ins, that's an indulgence on their part that can be revoked whenever it suits them. And that's fine, because it's their universe, not ours; we're just borrowing their toys and playing with them for a bit.