Apparently the head writers wanted Fields for Pamela but Bridget Dobson wanted Dusay. I found this on another soap forum:
From a 1988 interview with Bridget:
"Jerry and I had conceived Pamela many years ago but NBC did not want us to bring on another forty-ish character. But we kept pushing because we know it could be hot stuff." Once the powers gave in, movie queen Samantha Eggar was signed, but quickly got cold feet when the rigors of daytime grind were fully explained to her. Fellow British actress Shirley Anne Field was cast instead. "She was Ann and Chuck's first choice," reports Bridget. "Marj Dusay was my choice but we cast Shirley Anne because we wanted the head writers to be excited. To me, Marj is a great beauty and seemed to be the kind of person that CC (Jed Allan) would have once been attracted to. Ann and Chuck's perceptions were not marginally different from mine - they were very different."
Ironically, Field wound up delivering a tepid, colorless performance and was given the heave-ho after three months. The replacement? Marj Dusay. Perhaps just as ironically, Dusay came on board as nothing short of brilliant with each icy line reading revealing encyclopedias full of character. Clearly, the plot possibilities were endless and the likelihood that the actress would emerge at the forefront of Santa Barbara - much as Elizabeth Hubbard rules supreme as Lucinda on As the World Turns - must have seemed delicious to loyal watchers. Guess again. Dusay was on the unemployment line before she knew what hit her and the dandy part was foolishly tossed into limbo.
Dusay lasted only until the summer of 1988, then returned for a week in March 1991. When SB was canceled, she had this to say about the experience:
"I always thought Santa Barbara was a very good show and liked the work on it. But when I went in it seemed to be just going to hell in a hand basket. I truly believe they had their problems within the show, and they just more or less hired me to get rid of the character. Pamela was the baby of the Dobsons, and when they had their struggle [with NBC, which resulted in their outster from the series they had created] somebody had to take it over, and by then the character couldn't go anyplace. It evidently was so much politics, and if I had known that it would have helped me feel a lot better. I thought Pamela was going to be a wonderful character, and she might have been had the Dobsons stayed. It felt good to return, because there was something of real substance for me to play, and I was much more comfortable. I liked the way the Dobsons brought Pamela back and where she was coming from. It's a shame the show was canceled and I'm sorry for all my buddies there."