They chose Diana Rigg after Brian Clemens and co. met her. It was "fait accompli" according to an interview with BC. That was the reason - her screen test wasn't as good as Moira Redmond (who they were also considering). In the end they had to decide between a "new face" or Moira. They chose the new face.frank wrote: Diana Rigg was cast to replace Elizabeth Sheppard-she wasn't the original choice. That casting was in part due to the undeniable and rare chemsitry Rigg had with Macnee.
As for on screen chemistry, that took awhile. Watch The Murder Market for evidence. They're still writing for Honor; Diana is hesitant and seems very 'new', and Pat and Diana lack the same spark seen in later episodes. So if it's not in their first episode I can't see how that level of chemistry could have been seen in the selection process.
frank wrote:What aree the odds she could connect to the new actor to a comparable degree? Or Sheppard may had remarkable enough chemistry with the new guy that Clemens et al might have been willing to stick out with her. But then there is the issue if Sheppard would have even been cast in the first palce with this different actor playing the male lead.
The showrunners would want to cast 2 people who are not only suitable for the part but connect well with and play off each other as effectively as possible. A different actress who was in the running might have end up being the winning candidate. That actress may have worked out splendidly and there would then be no reason to bring in a replacement for the role of Mrs Peel.
We are told Elizabeth was let go because she lacked the warmth they were looking for. Brian Clemens has said that and so has Patrick Macnee. BC also said she - along with her out of work actor boyfriend/husband - was coming on set with scripts changed and other unwanted input. BC said if she was delivering on screen she would have been okay, but she wasn't - that's as close I can get to a quote without looking it up.
There's no mention of 'chemistry' (or lack of) between Pat and Elizabeth Shepherd as far as I can see.