Bit of a vague one I'm affraid. American gangster film set in the 20's/30's (I think). Can't remember anything about it except a psycho gangster hides in the back of someones car and cuts their throat. I thought the actor playing this psycho was Steve Buschemi but I've trawled his archive and can't find anything. Whoever played this character is little and weasley like Steve (no offence if you read this Mr. Buschemi)
I think there was a scene in Once Upon a Time in America that fits the bill (but i also seem to remember the ol' hiding in the back of the car and slashing the throat gag is quite a common thing in mob films!).