I 'get' what caused Gomez and Lense to be memorable guest-spot characters. And I can 'see' the process behind selecting them and the storytelling potential they possessed from the get-go. But, what traits did Duffy and Stevens exhibit during their episodes that made lasting impressions? Did editors know these characters had story potential or was it a matter of finding a few familiar faces and filling in the rest later?
Very little beyond their existence. Basically, we wanted to make use of some of the engineers we'd seen onscreen on both
TNG and
DS9. Duffy and Stevens had the advantage of a physical presence we could hearken back to but basically no character development so they might as well have been original characters.
When John Ordover and I were developing the series back in 2000, one of the things I did was go through the engineers we'd seen on both shows who a) had substantial(ish) roles, b) weren't dead, and c) weren't doing something else at the time. Also some were not used because they'd be too high-ranking, and we already had Gomez and Duffy for the two top positions on the S.C.E. team. Characters like the four chief engineers from the first season of
TNG were rejected because they'd be commanders or higher in the timeframe we were telling stories in.
Did you guys create the broad outlines of the Gomez and Corsi romance subplots first and then select the men (or vice versa) or were those plot lines created after Duffy and Stevens had already been selected?
Gomez and Duffy having been an item on the
Enterprise was John's idea from the very beginning. The Corsi-Stevens liaison was a last-minute thing I threw into
Cold Fusion as I was writing it just to see what would happen. I must admit to being pleased with the long-term results.
Were there any other on-screen characters considered for the CoE as regulars that were ultimately rejected? Are you able to share who they were and the thought process behind why they were not selected?
See above. We also considered Barclay for obvious reasons, since he's the non-chief engineer that has the most screen time, but he was being used by
Voyager at the time, so we couldn't really make much use of him.
When the time came to add Sarjenka to the mix how did that process come about? (Why the new character in the first place? Who thought of her from 'Pen Pals'?)
It's both Terri Osborne's fault
and Ilsa Bick's fault
at the same time!
Okay, the story that Terri told in
Progress was one she'd been wanting to tell for a while, as she felt there needed to be a followup to Sarjenka's storyline. She pitched it to me as a flashback story involving Gold's previous command, with the series hook being that we saw Gold on another ship and what led him to thinking the S.C.E. might be fun. That was one of three flashback proposals I'd gotten, which led me to deciding to do
What's Past, with the fourth being a
Vanguard crossover I asked Dayton & Kevin to do (the final two
WP stories were added later when the decision was made to move the reprints to trade paperback).
In any case, Terri was pootling along on
Progress, when Ilsa turns in the manuscript for
Wounds Book 2. Now a while back, Ilsa and I had been talking about the notion of Lense being pregnant because she'd make
such a lousy Mom, being the galaxy's biggest narcissist, and wouldn't that be fun? I said that was worth pursuing at some vague unspecified point in the future -- and then she drops the ending of
Wounds on me.
I thought about it for four seconds, and then I decided, what the hell, this could be fun.
However, this immediately affected two different stories. The first was Bill Leisner's
Out of the Cocoon, which was the eBook after
Wounds, in which Lense played a big part and which, oh yeah, Bill had already finished writing before Ilsa turned in Book 2.
Sheepishly, I e-mail Bill and tell him to do a rewrite, but it's not his fault... (Lense being pregnant actually wound up making OotC more compelling, which was a relief.)
The other story was Terri's.
Progress was going to end with Sarjenka going to Starfleet Medical. The
Sabre class of ships is not one that is conducive to having an infant on board, so I realized that we were going to need someone to take over for Lense once the kid was born. And it might be fun to bring said replacement in sooner as an apprentice, so to speak, and also create some conflict, since Lense won't want an assistant getting underfoot.
So I asked Terri what she thought about ending the framing sequence of
Progress with Gold and Gomez offering Sarj a place on the ship.
After she threw things at me (as this changed the focus of the story from explaining Gold's past to setting up Sarjenka's future), she tackled it nicely.

Her only proviso was that she get to write the story where she gets her memory back and confronts Picard about it, which she did in
Remembrance of Things Past.