At the time he was cast in TOS, Shatner was successful as a "leading man type," not so much a character actor.
A lot of actors still start out that way, and if they're lucky and have the chops they can move into a "character" type as they age. Not to say that Shatner doesn't have the chops, but he never really had to embrace it.
True. I'm just wishing that Shatner in particular and the TOS cast in general had had a little touch of
Dustin Hoffman's approach in
Tootsie. Not the cross-dressing thing, but the actor who is super-dedicated to his craft and wants to become a character rather than force every character to be himself.
While I'm far from a
Robert De Niro expert, I think he became famous in the 1970s by pouring himself the craft and almost disappearing into a role.
The ST films could have benefited from actors who were more dedicated and hard-working, and less vain and superficial. [And in the supporting cast, less rusty. It got to the point where some of them didn't need rehearsal, they needed lessons.]