A very interesting topic. I'm sure the directors had different temperments and actors had their favorites. If Lela Swift wanted the actors to project "big", certain actors found that appealing. Dark Shadows was larger than life and a certain amount of theatricality was fine, but overdoing it, as Grayson Hall sometimes did, didn't work for me.
As far as Joan Bennet goes, I agree with the general consensus that once BC came on board, and after the Jason storyline, her importance on the show diminished. More time was spent on the supernatural elements (and special effects) and she, as well as a few others, became props. The balance went in favor of those elements that were popular. Once we had time traveling vampires, ghosts, witches and werewolves, it was hard to get into a meaty domestic situation.
Frid certainly was overworked with the show and promotional work and it showed at times. When he was good, he was very good (as Mae West used to say!).