I never cared for Harry Landers performance as Dr Coleman in "Turnabout Intruder" (TOS). His delivery was so monotone and there was a scene where you're supposed
to realise that Coleman perhaps loved the totally deranged Janice Lester, but it really gets lost in the flatness.
Making the situation worse, many of Landers' flubbed takes are part of the Season Three audio tapes that were found in a Paramount trash bin and included on the infamous Blue Pear LP record, "Star Trek Bloopers".
I agree that Anthony Montgomery (ENT) must have really unimpressed the writers, because it seemed like they stopped even trying
to find him something to say or do, although in the episodes where he did have something meaty, he certainly didn't seem too bad.