Over the last century or so, movie quotations, like pop-music lyrics, have come to replace Biblical verses and Shakespearean couplets as our cultural lingua franca, our common store of wit and wisdom. Yet many of the most frequently cited motion-picture lines turn out to be misquotations. The speech from “Dirty Harry” in which Clint Eastwood says, “You’ve got to ask yourself one question: ‘Do I feel lucky?’ Well, do ya, punk?” is commonly shortened to “Do you feel lucky, punk?” Michael Douglas’s “Greed, for lack of a better word, is good” (“Wall Street”) is condensed to “Greed is good.” Expressions of James Cagney like “You dirty, yellow-bellied rat” (“Taxi!”) and “Dirty, double-crossing rat” (“Blonde Crazy”) are immortalized as the snappier “You dirty rat.”

Why do we so frequently get the lines wrong?

One phenomenon at work, as in the cases above, is compression. Even Bartlett’s Familiar Quotations falls prey to this type of error. It cites “Apocalypse Now”: “I love the smell of napalm in the morning. It smells like victory.” What Robert Duvall really says is: “I love the smell of napalm in the morning. You know, one time we had a hill bombed for 12 hours. When it was all over, I walked up. We didn’t find one of ’em, not one stinkin’ . . . body. The smell, you know that gasoline smell, the whole hill. Smelled like victory.”

Sometimes lines are altered so they can stand alone, without the cinematic context. In “Island of Lost Souls,” Charles Laughton remarks, “They are restless tonight.” Now we paraphrase this as “The natives are restless.” Sometimes a specific reference is changed to a generalized one. “If you build it, he will come” from “Field of Dreams” becomes “If you build it, they will come.” Misquotations often improve upon the screenwriters’ originals by offering a better rhythm or cadence. Thus “Win just one for the Gipper” (Pat O’Brien in “Knute Rockne, All American”) is remembered more mellifluously as “Win one for the Gipper.”

The most famous example of a film line improved by the popular mind is, of course, Ingrid Bergman’s request to the pianist in “Casablanca”: “Play it, Sam. Play ‘As Time Goes By.’ ” It didn’t take long for the line to begin to shift. Nigel Rees, the British author and quote maven, has noted that Jack Benny included “Sam, Sam, play that song for me again, will you?” in a radio parody of the movie a year later. At some point along the way, it became the memorable “Play it again, Sam,” which Woody Allen helped to cement by using the paraphrase as the title of a 1969 play and a 1972 motion picture.