"Harvey was one of the most compassionate and empathetic human beings I've ever met," Giamatti said in a statement.
Comic book writer Harvey Pekar dies at 70 in Ohio →
"If he needs to get deeper in the scene, he turns it inside out. It becomes like a Picasso painting, and you follow him. He knows what he is doing," Giamatti said of working with Hoffman.
Director: 'Barney's Version' took 10 years →