that particular line and Scorsese liked it and decided to keep it in, unless you have a copy of the script in front of you...

And as to the next question that I'm almost certain will be asked, William Monahan didn't just win the Oscar based on the dialogue alone, but also because he did a fine job of taking the basic storyline of Infernal Affairs and making it unique to Boston -- which includes dialogue, yes, but also includes locations and a working knowledge of how people in the city behave.
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freak out
in a moonage daydream
oh yeah