Dick York by far was the better Darrin Stephens. Dick Sargent did an admiral job replacing him but things were never quite the same after the 5th season. Dick York had better chemistry especially with Elizabeth Montgomery and Agnes Moorehead. Would have been interesting to see how Dick Sargent played off some of the earlier season actors like Marion Lorne or Alice Pearce, but I am pretty sure it would not have been as well as Dick York did. The romance was also more vibrant between Montgomery & York.
If Sargent had been the original Darrin, I wonder just how different things would have turned out....he always struck me as more of a "stiff shirt" for lack of a better description....always a little too much starch in him as compared with York who could run the gambit of facial expressions and emotions not to mention all his physical comedy as well.
I always felt there was a deeper love conveyed between Sam & Darrin with York's portrayl than with Sargent's.
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