Two things I did not know about Gillian Dobb (Agatha):
1. According to IMDB, she is dead! (Passed away in 2001, if that is correct), and
2. Her only acting credits are MPI and a Gidget movie in 1985 (again, true?)
Well anyway, I loved the Agatha character too.
According to the Magnum commentaries (Believe its on Resolutions I commentary on season
, Ms. Dobb was a local actor who was cast as an extra (and it sounds like she would have been little more than an extra from their initial POV - see "The Elmo Ziller Story"), but won the crew over with her ability and the part of Agatha was specially created for her - or at lease thats my take from Charles Floyd Johnson.
The Gidget movie, I believe, was set in Hawaii..... its "Gidget's Hawaiian Adventure" or something like that. So, it kind of makes sense that Ms. Dobb would only be cast in Hawaiian shows.
Too bad there weren't more of them for her, but she'll always be Agatha to me!