It's been the most consistent version of the story that the intention of Dan Curtis was to bring in a super bad guy spook, jack up the ratings, and then kill off the character. Obviously Curtis hoped for a good reaction to the character but the reaction he got was more than he bargained for.
Associate Producer Bob Costello has said he and the writers brought in Frid to talk with him about the role and he had the look they wanted. I have no doubt that the fact that Frid had gone to Yale with one of the writers, Ron Sproat, played a role in the hiring. Frid did a great many plays at Yale (though there as a directing student) and Sproat saw him. Yep, the old "who you know" could pay off even then.
Nancy
OMG Ringo that portrait is killing me!!!
I wonder if their original intention was that Barnabas was young and handsome. Curtis has stated that he chose Frid from a list of candidates simply because he was wearing a cape in his photo. At the time he didn't envision Barnabas being a romantic lead, but rather, the most frightening villain he could think of. Frid was probably the "scariest" looking of the actors he chose from in his own estimation.
(Although, in my mind, the thought of Bert Convy is much more frightening ;-) )