The STAR TREK films should never have given such important "ensemble" roles to the legacy supporting actors. They didn't work on their craft and it showed. They should have had cameos at most, with new, good actors in the real supporting roles behind Shatner, Nimoy, and Kelley.
Geez, y'know Doohan actually had acting chops before he resigned himself to hiding behind a brogue. I think he always worked on his craft when there was opportunity to do so
-- even post TOS he has a few credits.
Mentioning Andy Robinson upthread makes me wish somebody could interview the various folks who screentested for Decker on TMP -- I remember that both Robinson and Tim Thomerson from QUARK tested, as well as Frederic Forrest, who certainly would have imbued Decker with a lot more gusto than Collins (a kind of William Atherton lite, if you've seen Wise's zepplin movie.)