This Starlog interview from 1987 takes a look at how she helped shape Star Trek and, arguably (if you ask William Shatner), the most successful character on the show.
"I was Gene's secretary at MGM in 1963 on The Lieutenant when he created Star Trek," she details. "He asked me to read it in his early 1964. This was the very first Star Trek series presentation. I read it and said, 'I only have one question, who's going to play Mr. Spock?' He pushed a picture of Leonard Nimoy across the table. I knew Leonard because he had appeared in my first story on The Tall Man.
The proposal had many possibilities and was certainly exciting. Of course, you could never tell if it would sell and if somebody else would believe it, but i certainly did. The captain at the time was Robert April, who eventually became Christopher Pike and the ship was, as I recall the Yorktown. Mr. Spock was pretty much like the Mr. Spock who appeared in 'The Cage,' and the doctor was Dr. Bocce. The other character weren't as settled It was completely unlike anything on television."
(...) In "This Side of Paradise" [...] the crew beams down to a planet where they find an Earth colony still alive, despite the fact that they're being bombarded with radiation. The colonists are in perfect health, thanks to a series of spores which make them immune, peaceful and idyllic. The crew is also affected, even Mr. Spock, who begins to act quite human.
"'This Side of Paradise' was the rewrite that got me the job of story editor," Fontana explains. "The story and script were originally by Jerry Sohl, and featured Sulu as the love interest. The spores were contained in a cave, and therefore, weren't much of a threat. My idea, which I stressed to Gene, was that the story could be improved if spores were all over the planet and unavoidable, and have Spock as the love interest. Everybody said, 'Spock?' I said, 'trust me,' and Gene did. It was an enjoyable episode to write, because there were so many things you could get into which wouldn't work under normal circumstances. It also had wonderful actors. Leonard's always good, but Jill Ireland as Leila Kalomi was exquisite."
(...) Considered the main force behind the exploration of Spock's Vulcan heritage, Fontana charted the "Journey to Babel" which introduced Spock's parents, Sarek [...] and Amanda [...], as part of an alien diplomatic party aboard the Enterprise.
"That one came about because of the mention several times, in the 'The Naked Time' and 'This Side of Paradise' of Spock's parents. I said to Gene, 'We've talked about them, let's show them.' He told me to do it, and I came up with 'Journey to Babel.' John D.F. Black had mentioned Spock's mother without being specific , 'The Naked Time' and I mentioned the mother and father, without giving them names, in 'This Side of Paradise.' I sat down and created two characters, emphasizing the triangular relationship - the rift between Sarek and Spock, with Amanda positioned in the middle."
(...) While there were quite a few gems among the animated adventures, it is Fontana's "Yesteryear" which stands out as being most akin to the original show. In this episode , Spock must travel backwards in time trough the Guardian of Forever to discover why he no longer exists in the eyes of those in the present time frame. Arriving on Vulcan, he meets himself as a child and helps the young Spock through his rites of passage. The child is saved by his pet sehlat, but the beast is wounded in the struggle and, by young Spock's decision, must be put to sleep.
"'Yesteryear' resulted from my looking back at the things we had done in the series, and remembering the time portal from 'City on the Edge of Forever,' explains Fontana. I thought we could use that for a legitimate trip, but then have something happen so that Spock has to return to Vulcan to his childhood. We would probe into these characters and see the beginning of some of the trouble with Spock and Sarek, Amanda's problems back then, and part of what made Spock Spock. I also felt strongly about dealing with the death of a pet. It was a very serious thing for kids. We were trying to put across a lesson to children, that when it comes time for an animal to die, if he must go, it should be with dignity."