I Know This Much is True

Mark Ruffalo

Dominick Birdsey / Thomas Birdsey

Melissa Leo

Concettina Ipolita Tempesta 'Ma' Birdsey

Rosie O'Donnell

Lisa Sheffer

Archie Panjabi

Dr. Patel

Kathryn Hahn

Dessa Constantine

John Procaccino

Ray Birdsey

Rob Huebel

Leo

Michael Greyeyes

Ralph Drinkwater

Gabe Fazio

Shawn Tudesco / Dominick and Thomas Acting Double

Rocco Masihi

Young Thomas Birdsey

Donnie Masihi

Young Dominick Birdsey

Laura Esterman

Ruth Hood

Imogen Poots

Joy Hanks

Bruce Greenwood

Dr. Hume

Marcello Fonte

Domenico Onofrio Tempesta

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Annie Fitzgerald

Miss Hanker

Joseph Ragno

Henrey Rood

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Acahkos Johnson

Penny Ann Drinkwater

Jaiden Klein

Marie Sexton

Charlie Reina

Channy

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Violet Adams

Bunny Borsa

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Oliver Patnode

Otero

Matt Helm

Young Leo

Derek Goh

17-19 Year Old Dominick & Thomas Acting Double

Agatha Nowicki

Angie Constantine

Irene Muscara

Prosperina Tucci

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Adrienne R. Williams

Mrs. Birdsall

Tatanka Means

Nabby Drinkwater

Philip Ettinger

Early Adulthood Dominick Birdsey / Thomas Birdsey

Aisling Franciosi

Young Dessa Constantine

Juliette Lewis

Nedra Frank

Harris Yulin

Father Lavie

Sue Jean Kim

Dr. Yup

Michael Simon Hall

Stuart Feinstein

Gina Costigan

Mrs. O'Meara

Steven Cox

Boyfriend of Dr. Hume's Daughter

MorningStar Angeline

Nabby Drinkwater's Wife

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Al Linea

Abruzzi

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Richard Fitch

Mill Foreman

Amy Lynn Stewart

Merciful Mother

Guillermo Díaz

Sergeant Mercado

Marylouise Burke

Librarian

Brian Goodman

Al

Sandra Berrios

Social Worker at Hatch

Laura Silverman

Kristin

Christine Parks

Humanities Department Chair

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Morgan Sullivan

Office Buddy

Jake Robards

Landlubbers (voice)

Michael Genet

Thomas' Surgeon

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María Gonzalez

Ma's Oncology Nurse