The fact that you approach your original work and your tie-in work the same way doesn't really disprove Bob's point. I think most writers of both probably do-- and that's why some good writers of original fiction make bad tie-in writers, and some good tie-in writers make bad original fiction, because they approach it all in the same way, but they have different needs.
But my point was to demonstrate that original and tie-in SF don't
have to be dissimilar. As with most categories, there's an overlap in the Venn diagram. They can be very different, but they don't have to be, because neither one is a single monolithic kind of thing.
Well, all I can say is that WTC did not feel like a stand-alone story fitted into the Star Trek universe, but rather a story organic from the background.
Because it was conceived from the start as a Trek novel, like most of my books. However,
Titan: Over a Torrent Sea was largely a reworking of an unsold original spec novel of mine, and three of the alien characters I created for
Titan's crew in
Orion's Hounds were characters I'd previously created for an original project that fell by the wayside as my plans for my universe changed. Most writers recycle unused elements from one project to another.
Which is basically my point. Trek Lit isn't one singular style. We don't have quite the same freedom to interpret the universe that our predecessors back in the '70s and '80s had, because the canonical universe has become so much more thoroughly defined and explored, but we're still free to write with our own distinctive styles and sensibilities. Nobody would ever mistake a CLB novel for a David Mack novel or an Una McCormack novel. There's no "house style" -- we have as much freedom to be ourselves as the canon allows. So the difference between us and the '80s writers isn't in the approach to the writing, just in the amount of canonical material we're constrained by. The gaps that we can fill in with our individual creativity are narrower.
Typically, stand alone sci fi will tend to have a more direct connection between the 'rules' of the background and the plot, which tends to make the stories much more epic and grand, or else feel much smaller and more limited in potential, compared to tie-in fiction.
And "typical" isn't the whole story. People read too much into averages and norms. The average position of a car on a circular track is the exact center of the circle -- a point it never actually occupies. Averages are a fiction we invent to simplify things that actually have a lot of variation. You want to really understand something, look at the whole bell curve, not just the middle.
Considering how many books can be ruined by "Han Solo would never say that" vs how few original sci fi books are ruined by having terrible, terrible characters and dialog, I stand by my assertion that good tie-in fiction takes different skills than what makes for a good hard sci fi novel. You just may have gotten lucky by having a complete and overlapping set of said skills to pay the bills.
I think it's just a matter of research. Tie-in writing requires researching characters and stories well enough to write about them convincingly. Hard science fiction requires researching scientific phenomena well enough to write about them convincingly. Historical fiction requires researching historical events and eras and cultures well enough... and so on. It's all about doing the legwork. I don't want to claim my skills are more extensive than anyone else's. Maybe it's just a matter of what fields a writer is more invested in exploring or has more of an affinity for.