I wonder if part of what's intriguing about actors like Heflin is that they're lead actors doing what's traditionally considered "character" work in their acting?

I started referring to actors of this type as "character leads" to describe precisely that combination and unsurprisingly they are my favorite kind: not necessarily stars, especially in the Code era, but incredible if you got them at the center of a movie instead of somewhere on its supporting fringes (where you might still wind up remembering them best). Sometimes you even get more than one to a picture, which is a treat.