I rewatched Mean Streets the other day; the vitality of this exchange remains startling. I was almost afraid to find out if it really was improvised. Scorsese:
There’s a big scene at the beginning of Mean Streets with Keitel and De Niro in the back room of the bar. De Niro makes up this whole speech — it’s really a bravura performance — and it’s all improvised. That scene made the picture.
Part of me found De Niro’s accent distracting, but then I reminded myself this was 1973—I’ve absorbed decades of watered-down riffing, but this performance is a primary source.