Seymour Hoffman, 46, was found dead in a Manhattan apartment late Sunday morning, police confirmed, with a needle in his arm.
The actor's death left Stormare, who starred alongside in the 1998 Cohen brothers cult classic The Big Lebowski, reeling.
"We were like siblings; this is just bloody awful," Stormare said in a statement through his Swedish manager Håkan Krantz, according to the Aftonbladet newspaper.
Stormare, perhaps best known as the violent recluse in the Coen brothers Oscar-winning Fargo, befriended Seymour Hoffman in New York in the late 1990s.
They also shared the same manager and have stayed in touch over the years. As a result, news of Seymour Hoffman's passing has hit Stormare especially hard.
"This is worse than when James Gandolfini and Heath Ledger died. It's too hard to talk about," Stormare said in a statement.
Swedish film director Daniel Espinosa had worked with Seymour Hoffman on the upcoming film Child 44, before the actor quit the project last summer after a relapse into drug abuse.
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"It was a real shame and quite tragic," Espinosa told Aftonbladet, calling the late actor an "unbelievably nice person".
"He was clearly one of the greatest talents of his generation."