“The Descendents” screenwriters Nat Faxon and Jim Rash have finished filming their directorial debut, “The Way, Way Back,” which they’ve been shooting throughout the summer around Marshfield, Duxbury, and Wareham. The local drama is about a kid (played by Canadian actor Liam James) who works at a water park and must deal with family drama thanks to his mom (Toni Collette) and her boyfriend (Steve Carell). Rash, who wrote “The Way, Way Back” script with Faxon before signing on for “The Descendants,” tweeted on Saturday, “2 the cast & crew of The Way, Way Back on our last day of production, Thank U all 4 being a part of what truly has been a dream come true.” Young actor River Alexander, who also appears in the film, tweeted his own goodbyes. “definition of bittersweet: Sweet - 2b part of such a special film; Bitter -gonna miss this cast and crew SO MUCH.”