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Playing Against Type: This is the film that got Hoffman recognized from simply the flavor of the month star, to a master actor destined to be one of the Hollywood Greats.
Throw It In: The Signature Line of "I'm walkin' here!" was unscripted. Because of the low budget, they didn't have permits to close New York streets and had to film with hidden cameras. In that particular scene, a taxi ran the red light and almost ran over Dustin Hoffman and Jon Voight. Hoffman almost said "I'm actin' here" before remembering to stay in character (though does forget the accent and limp). Fortunately, this adds to the "welcome to New York" feel of the film, and hints there's more to Ratso than it appears.
There is some dispute over the scene, though. The director claims the taxi was driven by a stunt driver who was supposed to elicit such a reaction from Hoffman, while Hoffman claims it was all unscripted. Since, if Hoffman is to be believed, he's ad libbed every line in every movie he's been in, his claim should be taken with a grain of salt, however the fact that traffic wasn't closed and it was filmed with hidden cameras certainly gives more credibility to Hoffman than the director.
And considering Hoffman doesn't break character, but loses his accent and forgets his limp, it does lend to the theory that it was unscripted.
What Could Have Been: Bob Dylan wrote the song "Lay, Lady, Lay" for this film, but didn't submit it in time for inclusion in the soundtrack. It most likely would have accompanied the Scribbage-and-sex scene with Joe and Shirley.
Randy Newman's "Cowboy" and Harry Nilsson's "I Guess the Lord Must Live in New York City" were also written for the film but passed over for one reason or another.