Between 1964-65, Bill Shatner appears to have briefly experimented with a slightly lighter toupee than he was accustomed to wearing prior to this point. One example of this is the above photograph from the set of the short-lived CBS legal drama For the People. The 1965 photo was taken during the filming of an episode entitled "Act of Violence" - Shatner is talking with the episode's director Sam Wanamaker. Notice the surprisingly light skull-cap, like the strings of a violin pulled over the main "belly", in this case a balding crown.
Interestingly, Shatner's toupee is not exactly up to filmable standards in the picture - the real and the fake have ceased to blend and, assuming they are about to film, the attention of a hair-stylist is urgently called for. But assuming this is a rehearsal or an after-shoot chat, it is also very slightly possible that in this photograph, Bill Shatner is not wearing his frontal hairpiece (we believe that he may have worn a two-piece piece) a fence-like structure that was glued onto his forehead to thicken up the front of his head.
Perhaps that is why Shats looks a little uncomfortable - the presence of the photographer catching him during a poorly-touped moment. This would also mean that at this point, Bill Shatner still had his own far thinner (and thinning fast) mini-frontal swoosh, which was then enhanced with a lace piece during filming. But it may be equally possible that he is fully touped up as usual in the photo.
A clip (the only clip we could find) of For the People shows Bill Shatner wearing the regular thicker toupee, both with his frontal lace swoosh and thicker skull cap:
Perhaps the clip is from an earlier episode, or perhaps the above photo strongly differs from how Bill Shatner was ultimately filmed for "Act of Violence" - it is difficult to say. Anyway, For the People was thankfully cancelled after only thirteen episodes. Thankfully? Well, shortly after, a gentleman called Gene Roddenberry called Bill Shatner on the telephone - and the rest is history.
But the toupee experiment apparently continued, this time definitely in front of the cameras too. Compare the very light toupee skull-cap in the top photo with what Bill Shatner wore in the second Star Trek pilot (his first episode as Kirk):
The two are very similar (albeit in the Star Trek pilot, he is definitely wearing his frontal piece too). Yet, by the time of the first regular Star Trek episode "The Corbomite Maneuver" filmed a year later, the classic "Jim Kirk" lace look was back to the slightly thicker form of earlier years...