This dates back to the almost-weekly appearances of George, Nichelle and Walter (and often Jimmy) at fan-run conventions in the late 70s and early 80s. You get the first hints of their growing popularity with fans from books such as "The Making of the Trek Conventions", "USS Enterprise Officers' Manual" and fan articles in "The Best of Trek" paperbacks. De, Majel, Gene and Rod Roddenberry were seen far less often and it was rare to get Shatner or Nimoy at a convention. The promise of "Phase II" stirred up fan thoughts of what might be in store for "the gang of four". Grace Lee Whitney bobbed up not long before Gene Roddenberry made a fuss about inviting her to return for ST:TMP. The actors were often on panels and had great repartee with fans, who helped them come up with approvals for first names (Walter Sulu, Penda Uhura), and reminisces of the characters' backgrounds (some of Uhura's from the "Star Trek Logs" TAS adaptations), which meant that minor references in TOS gained notoriety when appearing in Trek novels of the 80s.
I recall seeing an explosion in interest in Klingons with ST III, and the ease of buying no-bake "dipping latex" for making forehead appliances easily. Of course, the original premise for that film called for Romulans, and a cloaked Romulan Bird of Prey.
Having met George Takei numerous times in the 80s and 90s, and Walter once, I can vouch that the Shatner stories were originally told with very little malice. Ditto Grace Lee. Comedy effect during con chats, told with nostalgic affection. But... as each actor started working on their autobiographies, the stories did get sharper, with more barbs and some resentment expressed. (Ditto Yvonne Craig, who told similar stories to Grace Lee's but with some anger/resentment, especially Shatner's tendency to cross his eyes in closeups and ruin his leading ladies' takes in romance scenes, meaning that his reaction shots got used.) Autobiographies without any "dirt" for the publicists to spin are a hard sell.
FASA had a boxed RPG supplement about Klingons, written by John M Ford (novelist of "The Final Reflection" and "How Much for Just the Planet?") and incorporating his world-building from those Pocket Books. ST III also spawned the "Klingon-English Dictionary", supposedly written by Marc Okrand in consultation with Maltz.
That is actually something I have heard / read very little about - what changes occurred during TSFS, and what was the original outline?