THE TIME BEFORE STAR TREK!

"What did the main actors on Star Trek TOS do after the show was cancelled but before the movies were made? The ten years between the end of the STAR TREK series in 1969 and the release of STAR TREK: THE MOTION PICTURE were pretty lean times for most of the cast.
For a time William Shatner found himself nearly destitute, living in a camper and taking any and every job he could, including episodic guest parts, game show appearances and TV commercials. For a while he returned to Canada to do guest roles in Canadian series. I have to say, it was a bit of a turn watching Captain Kirk threatening to blow up a bank in Toronto. He also did a few B movies, such as KINGDOM OF THE SPIDERS (1977), BIG BAD MOMMA (1975) and THE DEVIL’S RAIN (1974), and starred opposite Doug McClure in the entertaining, but short-lived, TV series THE BARBARY COAST (1975–1976). However, you have to give the man credit, whatever the quality of the role, Shatner kept working. In the period between 1969 and 1979, DeForest Kelley, Nichelle Nichols and Walter Koenig barely worked at all, appearing in a relative handful of one-shot supporting roles. Typecast as “Scotty”, James Doohan also struggled to find work, but his time on STAR TREK did net him a regular role as the Commander of the Space Academy on JASON OF STAR COMMAND (1978).
George Takei, being at the time one of the few, but growing, number of established Asian actors in Hollywood, fared better in landing roles that required an ethnic actor, appearing consistently in guest roles on episodic television and in supporting parts in feature films.
And, of course, the entire original cast reunited for the animated STAR TREK series in 1973–74.
Of all the cast, Leonard Nimoy undoubtedly fared the best. As others have noted, immediately after the cancellation of STAR TREK, Nimoy starred in two seasons of MISSION: IMPOSSIBLE. Afterwards, in short order, he starred in the pilot for the unproduced series BAFFLED! (1972), continued to appear on television in guest roles, and played the lead role on stage in productions of FIDDLER ON THE ROOF (1971), ONE FLEW OVER THE CUCKOO'S NEST (1974) and SHERLOCK HOLMES (1975).
As STAR TREK: THE MOTION PICTURE was gearing up for production, Nimoy had just co-starred in the 1978 remake of INVASION OF THE BODY SNATCHERS, and was hosting the oddly compelling paranormal phenomena documentary series, IN SEARCH OF... (1977-1982), which I vividly recall watching every Sunday night.
And then, of course, came STAR TREK: THE MOTION PICTURE and all that followed…."