Actually, Shatner did a fair amount of directing, but he had plenty of other things keeping him busy, so never went into it as much as Nimoy. Interesting story about Nimoy - being no fool, he knew they couldn't do a "Search for Spock" movie without him, so his asking price was not his fee, but rather that he got to direct. From the success of that film, he went on to non-Trek directing gigs, most notably "Three Men and a Baby."
Shatner directed eight episodes of T.J. Hooker though, as well as a couple of random tv show eps ("Kung Fu"The Legend Continues," "Perversions of Science") plus the tv movie version of his Tekwar novels, and several eps of the subsequent series. He also directed a horror film called "Groom Lake," and as we discussed last month, originally was going to do that "Fire Serpent" film too. Plus of course he's directed extensively for the stage in his youth, especially in the couple of years immediately after Trek.
I think the main reason he didn't go further into directing is that he was still viable as a leading man, and had a very lucrative gig as the host of whatever that cop documentary series was. Not "America's Most Wanted," but one like that. Ran for years, I just can't think of the name, and I don't see it at the IMDB. And then he got into writing novels too, which is a full-time job for many.
It's funny - in revent years, the 5th Trek movie is coming to be appreciated a bit more. It would have fit in very easily with the original series, with Abraham Lincoln and the God Apollo appearing in space, and the crew fighting Jack the Ripper, Wyatt Earp, and Gorgon the Friendly Angel.
It actually reminds me a great bit of the cartoon series. If you turn off the volume, and just look at it, it's visually one of the best. And while a lot of the actual plot doesn't quite work... a good bit does: an aging Kirk climbs mountains to prove himself to himself, a Vulcan uses his mind-control powers for something other than pacifism, the Enterprise crew does a land assault straight out of the Horatio Hornblower novels, Sulu cockily attempts a very realistic dangerous shuttle maneuver, Uhura is still sexy and still singing past age 50, etc. At this stage, Shatner was the key to the franchise continuing (he too had parlayed his participation into a writing/directing gig) and I suspect no one at Paramount had the nerve to tell him his script needed a re-write - with the tv series flourishing (and probably plans for DS9 already on the drawing board) and the success of the previous 3 films, they probably thought it couldn't fail, rather than demand a re-write by someone else. Although we must note that Shatner only co-wrote it - Harve Bennett, who cowrote and produced 2, 3 and 4 is just as responsible, if not moreso (since he was the boss.) I bet it could have been revised into a very effective film. The guy who played Sybok, for example, Laurence Luckinbill, is a
serious Broadway actor whose career I've followed since I was a kid (because he was married to Luci Arnaz. :laugh: ) who could have done just about anything. And that opening sequence with the starving guy in the desert is visually stunning.