Most of what follows comes from https://en.m.wikipedia.org/wiki/Mare_of_Easttown and also the trivia section of the IMDB entry on Mare of Easttown
Critics and viewers have praised the series and Winslet for convincingly replicating one of the varieties of the Philadelphia regional accent, an oddity in mainstream media. The specific accent is known as the “Delco accent”, after Delaware County to the west of Philadelphia.
After three episodes, Saturday Night Live created a segment called ‘Murder Durder’, spoofing the show’s Philadelphian accents and plot points. The cast and crew found it hilarious and were very flattered
One of the only productions to attempt the Philadelphia accent, much less master it. The cast received universal acclaim for their portrayal of the harsh Philadelphia sound; notoriously known as one of the hardest accents ever to imitate. The production was so committed to having authentic accents that dialect coach Susan Hegarty was the second person hired after Kate Winslet. The vowel sounds are so wildly inconsistent from word to word that linguistic professor Betsy Sneller suggested that it is easier to go over each word individually. “It’s really, really difficult to learn even in a natural language setting, so the fact that Kate Winslet [and the cast are] doing it is very, very impressive.” Craig Zobel recalled that Winslet would show up to set with her script completely covered in markings of the specific vowel sounds.
All 7 episodes were shot at the same time. Craig Zobel had to continually check with the script supervisor to figure out what part of the story the episode was, and what certain characters knew. When production was halted due to COVID, a significant amount of rewriting had to be done, to limit the amount of people on set. Zobel credited Kate Winslet with guiding the production on how to maintain a safe work environment, after consulting with Scott Z. Burns, following their collaboration on the pandemic film Contagion (2011).
Production began in October 2019 and continued until March 13, 2020, when COVID restrictions forced filming to cease, mere weeks away from completion. In what was originally envisioned as a six-week break, production was delayed for several months until September. For the cast, they had to continually rehearse their accent, for fear of losing the specific Delco accent. When filming resumed, the scenes took almost double the time to film, due to the extra precautions required. Craig Zobel estimated that 50% of the script had to be rewritten in accordance with social distancing guidelines.