I Know This Much Is True (miniseries)

• Mark Ruffalo as Dominick and Thomas Birdsey
• Melissa Leo as Ma, Dominick and Thomas' mother. It is later revealed her full name is Concettina Ipolita Tempesta Birdsey.
• John Procaccino as Ray Birdsey[7], Dominick and Thomas' stepfather
• Rob Huebel as Leo[7], Dominick's best friend
• Michael Greyeyes as Ralph Drinkwater, a former classmate from Dominick and Thomas’ youth whose life intersects once again with Dominick[7]
• Gabe Fazio as Shawn Tudesco, a weightlifter down at hard-bodies. Fazio also played Dominick and Thomas Birdsey as a stand-in for scenes where both of them would appear.
• Juliette Lewis as Nedra Frank, a self-absorbed grad student hired by Dominick
• Kathryn Hahn as Dessa Constantine, Dominick's ex-wife
• Rosie O'Donnell as Lisa Sheffer, a social worker for unit two at the Hatch Forensic Institute
• Imogen Poots as Joy Hanks, Dominick's live-in girlfriend
• Archie Panjabi as Dr. Patel, Thomas' newly appointed psychologist
• Philip Ettinger as Young Dominick and Thomas Birdsey[7]
• Aisling Franciosi as Young Dessa
• Bruce Greenwood as Dr. Hume[7]
• Marcello Fonte as Domenico Onofrio Tempesta, Dominick and Thomas' maternal grandfather from Sicily
• Harris Yulin as Father LaVie

• Donnie Masihi as 8-Year Old Dominick Birdsey
• Rocco Masihi as 8-Year Old Thomas Birdsey
• Joseph Ragno as Henrey Rood
• Laura Esterman as Ruth Rood
• Annie Fitzgerald as Miss Hanker
• Irene Muscara as Prosperine Tucci ("The Monkey"), Ignazia's best friend and lover, loathed by Domenico Tempesta.
• Matt Helm as Young Leo
• Agatha Nowicki as Angie Constantine, Dessa's sister
• Tatanka Means as Nabby Drinkwater
• Achakos Johnson as Penny Ann Drinkwater

• Brian Goodman as Al ("One")
• Guillermo Diaz as Sergeant Mercado ("One")
• Laura Silverman as Kristin ("One")
• Joe Grifasi as Steve Falice ("Four")
• Felix Solis as Nurse ("Four")
• Sue Jean Kim as Dr. Yup ("Four")
• Roberta Rigano as Ignazia Tucci Tempesta (Violetta d'Annunzio), Dominick's grandmother from Pescara. ("Five")
• Simone Coppo as Vincenzo Tempesta, youngest brother of Domenico Tempesta. ("Five")
• Zaria Degenhardt as Concettina Tempesta, Dominick and Thomas' mother as a young girl ("Six")

In June 1998, it was announced that 20th Century Fox had acquired film rights to I Know This Much Is True by Wally Lamb, with Clinica Estetico producing, and Jonathan Demme potentially directing.[15] In July 2000, it was announced Matt Damon would star in the film, with Jim Sheridan directing from a screenplay by Richard Friedenberg.[16] In July 2004, it was announced Gina Prince-Bythewood would direct and re-write the film.[17] The film rights expired and reverted back to Lamb, who thought the novel would be better adapted into a miniseries rather than a film. Being a fan of Mark Ruffalo's work, Lamb suggested Ruffalo should play the role of the twins.[18][19] Ruffalo was sent the book and wrote an e-mail to Lamb confessing his love for the novel, stating he definitely wanted to be involved.[20]

Ruffalo had been interested in working with Derek Cianfrance, and reached out to see if Cianfrance would be interested in directing and writing the series.[21] Lamb told Cianfrance and Ruffalo to make the material their own, and did not ask to see the scripts.[22] In October 2017, it was announced HBO would produce and distribute the series, with Ruffalo starring and executive producing, Cianfrance directing writing, executive-producing alongside Lamb, with FilmNation Entertainment producing the series.[23] In October 2018, the series was greenlit.[24]

Ruffalo first shot all of his scenes as Dominick, requiring him to lose 15 lb (6.8 kg). After finishing his scenes as Dominick, Ruffalo took a six-week break to gain 30 lb (14 kg) and then shoot his scenes as Thomas.[25] Gabe Fazio portrayed the roles of both Dominick and Thomas. He is the same height as Ruffalo and lost thirty pounds to play Dominick when Ruffalo shot his scenes as Thomas.[26] Production concluded in October 2019.[27]

In April 2019, Melissa Leo, Rosie O'Donnell, Archie Panjabi, Imogen Poots, Juliette Lewis and Kathryn Hahn joined the cast of the series.[28] In November 2019, it was announced Aisling Franciosi, John Procaccino, Rob Huebel, Philip Ettinger and Michael Greyeyes had joined the cast of the series.[29]

The members of the Psychiatric Security Review Board in Episode 4 are played by some local officials in Dutchess County, New York, where most of the series was filmed: State Senator Sue Serino, sheriff Butch Anderson, and his wife Danielle, president of the Mid-Hudson Civic Center in Poughkeepsie.[30]

Principal photography began in April 2019, in New York's Hudson Valley.[31][32][33] On May 9, 2019 a fire erupted on the set of the series at a used car dealership in Ellenville. The building, film equipment and 20 vintage cars were destroyed. There were no injuries but it put filming on hold.[34]

Poughkeepsie cityscape with Mid-Hudson Bridge, similar to view used in series

Poughkeepsie was used for many of the scenes set in Three Rivers, with the Mid-Hudson Bridge visible in the background of the cityscape in most establishing shots. The Birdsey family home is at the corner of Delafield and Hoffman streets. The Vassar College campus is used for scenes set at the University of Connecticut, and Dr. Patel's office is in a strip mall on U.S. Route 9 south of the city. A flashback scene in a later episode required covering Mount Carmel Avenue with dirt to recreate a pre-automotive era. The casino in the closing scenes of the series is the new patient pavilion at Vassar Brothers Hospital, still under construction while it was used in the production.[30]

Elsewhere in Dutchess County, Dominick and Joy's apartment is at Pleasant Valley Estates, and he and Leo play their racquetball match at a Fishkill club; Seasons restaurant in that community was also the location for some scenes. The flashback to 1922 was filmed at the Market Street Industrial Park in Wappingers Falls, where the costumes and sets were stored as well. Many of the mental hospital scenes were filmed at the Taconic Development Disability Services offices in Wassaic.[30]

Across the Hudson River, Thomas's initial breakdown in the opening scene was filmed in the Newburgh library, looking out across the river, and Dominick and Dessa are shown entering the terminal at Stewart International Airport near that city in flashback, where she boards a flight for Europe. In addition to the Ellenville car dealership used for the scenes with Leo, some other locations in Ulster County were used, including Awosting Falls in Minnewaska State Park Preserve on the Shawangunk Ridge west of New Paltz. The Springtown Truss Bridge on the Wallkill Valley Rail Trail across the Wallkill River north of that village, also known for its use in the opening scenes of the 2018 horror film A Quiet Place and its unreleased 2020 sequel, was used for the flashback scene where Dominick and Thomas's grandfather throws a bag with a live monkey into the river.[30] A section of U.S. Route 209 outside Kingston also was used for a confrontation between Dominick and Thomas in the first episode; the road was closed for two days in both directions for filming.[35]

On Rotten Tomatoes, the miniseries holds an approval rating of 76% based on 50 reviews, with an average rating of 6.85/10. The website's critical consensus reads, "I Know This Much is True's relentlessly grim plotting makes it a difficult watch, but the strength of Mark Ruffalo's dual performances is enough to make it a dark tale worth tuning into."[36] On Metacritic, it has a weighted average score of 66 out of 100 based on 25 reviews, indicating "generally favorable reviews".[37]

Writing for The A. V. Club, Ines Bellina rated the series a B–, praising Ruffalo's performance but criticizing it for not being enjoyable to watch and summing it up as "an uneven journey, overwhelming in its self-indulgent trauma... [but] Against all odds, it ends on a more heartfelt and hopeful note than the preceding six hours would lead you to believe."[38]