The role of Dr. Hoffman, as originally written and portrayed in the early episodes, simply could NOT have been sustained for much longer than it was, in my opinion. She was strident and relentless and completely lacking in subtlety. She spoke down to every other character with which she exchanged dialogue, and her physical gestures were abrupt and graceless. I assume this was the portrayal agreed upon by Curtis, Hall and the directors......and it was powerful indeed.
No audience would have tolerated it for very long. It was so extreme as to border on caricature. But it did establish Dr. Hoffman's inteligence, her self-confidence, her fearlessness, and her total commitment to her science. There could be no doubt that this woman was unafraid of the "dark", and would stare evil in the face with nothing less than contempt.
But of course, "something else" had to come next. Julia Hoffman needed a woman's soul as well as a brilliant mind. She needed an object of affection, and preferable one that was worthy of someone as complex and deep as she. Who better than a man from a different time, who's existance was utterly without precident, and who would choose to place himself completely at her mercy?
I don't think Julia ever really surrendered to Barnabas at all. On the contrary, I think her behaviour was so protective of him because HE had surrendered his secret and his safety to HER.
I think it fair to say that the character of Julia Hoffman was, in fact, the most carefully and precisely drawn of all the characters on the show, and I believe Grayson Hall's performance indicates she thought the same thing.