True.Therin of Andor said:
When actor's performances excite the writers, the writers start thinking of new scenes to write for them. So good scripts/scenes lead to good performances and good performances lead to good scripts/scenes.
If Shran and Tholos had been forgettable characters in "The Andorian Incident", the Andorians may have been one-hit wonders for ENT. Ditto Q in "Encounter at Farpoint" (TNG).
David Gerrold wrote "The Trouble With Tribbles" (TOS) assuming he was writing a return episode for Kor. Ditto Jerome Bixby and "Day of the Dove". John Colicos was unavailable both times. But in both cases, the performance of Colicos had inspired them.
Horizon had the potential to be a good episode if Montgomery had the acting chops to do serious scenes. The man is too damn wooden, and not Robert-Beltran-when-he-doesn't-feel -like-doing-a-good-job wooden.
Also, what you said makes me wonder why the writers of ENT didn't do more with Phlox, since he was probably the best of the cast that wasn't the Big 3.
But that's neither here nor there.