Terry's British accent at the end
I think Victoria Tennant should've attempted an American one, since that was Edwina's real accent. It doesn't make sense that Terry's body would continue to have an accent, if it was now Edwina who was operating her. When Edwina was inside Roger and momentarily took control of his body/voice (e.g. court room scene), Steve martin gave off the mannerisms and inflections of Edwina, which made you believe she really was in him. But Terry's body still acted and sounded the same after the final transfer, so I wasn't convinced she was really Edwina now, if it weren't for the final shot of Martin and Lily Tomlin dancing in the mirror.
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