Jonathan Frid himself has said he is a slow study. He tended to have a lot more lines than the others because Barnabas was the lead of the show. When Selby came on, it helped alleviate the pressure because there was another male lead to take over and give JF a bit of a break, but the episodes in which he appeared still tended to be very dialogue heavy.
Despite these innate difficulties, there were several scenes on DS where I thought he was just brilliant. I think his secret was as Jean-Claude has said, his belief in who and what he was playing. He projected a tremendous integrity as Barnabas, which probably had a lot to do with the unusual attractiveness of a middle-aged courtly gentleman to America's teen-agers during the "youthquake" of the Sixties. Frid's integrity goes a long way towards making Barn palatable to me now, since so much of what the character actually does is, to be tactful, rather unsavory.
G.