John Lithgow won an Emmy for his terrifying work as Mitchell and was undoubtedly a crucial role in the show, a foil for Dexter who could apparently balance his double life as a serial killer with his role as husband and father. By killing Rita, however, Trinity Killer – a violent man against his own family – put an end to Dexter’s dream of “having it all” as both killer and family man.
We can only speculate on how Arthur will return to the show, but Lithgow apparently only works about a day on the set per. Deadline. Therefore, it seems likely that the character appears only as a hallucination or as another voice in Dexter’s head, similar to how Harry became a recurring presence. It would make sense if the man who killed Dexter’s wife continues to haunt him beyond the grave, even though his presence is only in his mind.
Viewers will find out exactly how Arthur Mitchell returns to “Dexter” when the revival premieres in autumn 2021.