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I can't imagine Vince Vaughn or Jason Schwartzman in the lead role either. But Wahlberg with a lisp would be the worst.
Early on Vince Vaughn was offered the role of Donnie, but he turned it down as he felt he was too old for the part. Mark Wahlberg was also approached, but he insisted that he should play Donnie with a lisp. [...] Development progressed in early 2000, when actor Jason Schwartzman expressed an interest in the script and agreed to play as Donnie. Kelly said this moment "legitimized me as a director" and recalled "all of a sudden people came out of the woodwork, it was alive again".
I feel like schwatzman could have done it, but the final casting was great. He nailed the demeanor of someone that's going through a bunch of inner turmoil