So kinda a weird opening, but here goes nothing. One of my two main projects is a character-driven sci-fi, and I am struggling to create and flush out the crew of the MC's ship. I don't have much experience in creating secondary casts, as my normal focus is a series of connected short stories where all the characters take turns being the MC. Now, I am faced with writing 6-10 characters who are not nor will ever be the MCs of the story. I guess I'm looking for tips and tricks on how to create a compelling cast of secondary characters without shifting the focus always from the MC too much. I want each to standout in their own way, without being limited just to their role (doctor, pilot, etc) but keep the reader's focus on the MC and his struggles. I hope this makes some kind of sense.
My advice would be to sort of approach it the way you always have, only don't write the stories about the secondaries. Go ahead and think out their characters as if you were going to, but you don't need to go into as much depth. Of course I say this without having approached it that way myself. Another thing you could think about is the character web—whatever your main theme is, each character has a different take on it. If you've got ten characters besides the MC, some of them might be tertiaries rather than secondaries.
You might consider giving each secondary character some definable characteristic that borrows from character tropes: the manic one, the mean one, the smart one, the neurotic one, the femme fatale, the bad boy, the wise elder, the geek/nerd .....
