When the stage has been properly set, the simplest physical gesture can be charged with meaning in a film. Bogart's nod in the Marseillaise scene in Casablanca stands out as perhaps the most striking example of this important resource in cinematic storytelling, and one particularly deserving of a closer look.
Rick (Humphrey Bogart) and Victor Laszlo (Paul Henreid) are upstairs in Rick's office, with Laszlo offering to buy the letters of transit. Rick refuses, and in reply to Laszlo's question as to why, Rick tells him to ask his wife. They then hear German officers singing Die Wacht am Rhein in the main room below. Rick and Laszlo go out on the balcony and look down at the Germans singing. Renault is watching from the bar, his eyebrow raised. Laszlo, listening tight-lipped, finally walks down the steps and goes decisively over to the band, telling them: "Play the Marseillaise! Play it!" The band members look down, then up toward Rick, who nods to them. Having obtained Rick's approval, the band then begins to play the Marseillaise, and one of the most electrifying scenes in film history unfolds.
Earlier nods in the film
On meeting Rick for the first time, we see two things he does before we actually see his face: 1) approving a customer credit slip handed to him by an employee and on which he writes "OK Rick," thereby identifying him for us as the owner of the café and defining him as the man in charge, the one the people working at the café go to when approval is needed; and 2) playing chess with himself, suggesting an enjoyment of strategy, intellectual challenge and self-sufficiency (in a positive sense). Soon after we see Rick's face and the intensity of his involvement in the game, he looks up and sees another of his employees, Abdul, on guard at the entrance to the room and asking with a glance whether the couple standing in the doorway may be admitted. Rick nods yes. After they enter, another person appears in the doorway and again Abdul, now with a sneer on his face, looks to Rick for a signal. This time Rick nods no, and when the man protests, Rick walks over and - as Ugarte (played by Peter Lorre) slips in - Rick confirms that the guest is unwelcome in this part of the café and lucky that his money is good at the bar. In the ensuing dialogue between Rick and Ugarte, we learn to our delight that the man Rick had excluded was a representative of the Deutschebank.
Nodding yes or no to an employee looking to him for a signal, is one of the first things Rick does in this film, and Bogart's nod in the Marseillaise scene is therefore grounded in our experience of Rick from the very start. Though the relation of the earlier to the later nods is hardly one of set-up to pay-off,  the earlier nods nevertheless help to prepare us for the later one, by defining Rick as the one who calls the shots.
The significance of the nod in the Marseillaise scene
1) Marking a new stage in Rick's development
Rick's overall evolution, including what we know of his past and can foresee of his future, can be divided into three periods:
an early idealistic period, when - as both Renault and Victor Laszlo point out - Rick ran guns to the Ethiopians and fought on the loyalist side in the Spanish civil war, earning himself a place of honor on the Nazis' blacklist;
a central period, filling most of the present of the film, characterized largely by a cynical and selfish neutrality, as expressed by the line spoken twice by Rick - "I stick my neck out for nobody"; yet even here, there are flashes of profound integrity, as when Rick tells Ferrari (Sidney Greenstreet) that he doesn't buy or sell human beings; presumably, Rick's fall into cynicism was triggered by what he experienced as a betrayal at the Paris railroad station when he received Ilsa's farewell note;
a final period, in which Rick overcomes his selfish and self-pitying stance and returns to the fight against oppression.
In the dialogue between Rick and Victor Laszlo just before the Marseillaise scene, we are reminded that Rick is at present squarely grounded in his neutrality stance, telling Laszlo for example: "I'm not interested in politics. The problems of the world are not in my department. I'm a saloon keeper."
Yet moments later, when the boundaries are clearly drawn between resistance and oppression, and the possibility of delivering Victor Laszlo's liberating response to the German song is dependent on a choice that only Rick can make, the saloon keeper risks everything and nods yes. As one commentator wrote:
The die is cast. At Rick's behest, a line has been drawn between good and evil in a place where moral ambiguity, also at Rick's behest, has been the order of the day.
The Rick the band members knew was the one who had stood by passively as Ugarte was arrested, and who consistently put the interests of the café above politics. This is why, when confronted with Laszlo's command to play the Marseillaise in defiance of the Germans, the band could not take it for granted that Rick would allow them to comply.
If any moment in this film might be called a point of no return, this is it. Here, for the first time, in nodding his approval, Rick takes a stand against the representatives of the Third Reich, and places himself on the side of resistance.
All of this is in the nod, which marks Rick's transition from neutrality to commitment. It is here that the ground is broken for future moves Rick will undertake, such as devising and carrying out a plan for getting Victor and Ilsa out of Casablanca, ultimately shooting Major Strasser in the process, and going off to join the Free French in Brazzaville along with Captain Renault, who - inspired by Rick - undergoes his own parallel conversion from neutrality to commitment. Renault's line "Round up the usual suspects" in the final airport scene plays the same point-of-no-return role in his development as the nod does for Rick in the Marseillaise scene.
2) Status and power
Paul Henreid did not want the part of Victor Laszlo when he was first assigned the role as a contract player at Warner Brothers. His initial response was that the script was terrible and he didn't "want to be the second lover in a film, second to Humphrey Bogart!" But he allowed himself to be talked into the role, provided among other things that he get Ilsa at the end, as befits a leading man. In other words, from the very start, he experienced a fundamental rivalry with respect to Humphrey Bogart's Rick.
This feeling of rivalry was dramatically reactivated when Henreid learned whom the band members look at before beginning to play the Marseillaise, as the following passage in Henreid's autobiography makes abundantly clear:
I am described by the Germans as a great leader of the masses, a man who can command obedience. That's the reason the Germans don't want me to leave Casablanca, and it's also the plot hinge. There's a scene in Rick's Café, one of the high points, when I order the band to play "La Marseillaise" to counter the Germans' singing "Die Wacht am Rhein," a very patriotic military song. The musicians look away, then back to me before they start playing, and I conduct them, singing myself.
After the rehearsal, I asked Curtiz, 'What the hell is going on? Why do they look away and then back at me?"
"Oh, yes," Curtiz said, "That - I told them to look at Bogie. I'll have a cut of Bogie nodding, giving them the order to play."
"But why?" I asked, confused.
"Because in the picture Bogey pays their salary, and they don't want to do anything that could get them fired."
"But for heaven's sake," I protested, "I'm supposed to be a leader of the masses, and here I have a stinking little band, and I can't get them to do what I want!"
Curtiz laughed. "Oh, it'll be all right. It will establish that Bogie is on your side."
So much for the relative status of Laszlo and Rick in this scene, as experienced from Paul Henreid's perspective, as well as the manner in which Curtiz pacified Henreid.
But there is another hierarchical relationship in play here as well: namely that involving Curtiz and Bogart, the latter being just as unaware as Henreid had been as to exactly what happens when Laszlo orders the band to play. And in this connection, it is ironic that the very nodding shot that invests Rick with so much power in the scene was directed in such a way as to make Bogart feel as powerless as possible:
One day, when Bogart appeared for shooting, Curtiz told him, 'You've got an easy day today. Go on that balcony, look down and to the right, and nod. Then you can go home.' 'What am I nodding at?' Bogart asked. 'What's my attitude?' 'Don't ask so many questions!' Curtiz replied. 'Get up there and nod and then go home!' Bogart did as he was told, and didn't realize until long afterward that that nod had triggered the famous 'Marseillaise' scene, where Henreid leads the nightclub orchestra in drowning out some Germans who'd been singing 'Die Wacht am Rhein.' It's a scene that, ever after thirty years, prickles the scalp and closes the throat, and for all Bogart knew he was nodding at a passing dog. 
There was no artistic justification whatsoever for holding back from Humphrey Bogart the shred of information he requested. In not letting him in on the meaning of the nod and instead insisting on blind obedience, Curtiz indulged in an arbitrary exercise of power at the expense of an actor who merely wanted to understand what was happening.
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Benchley, Nathaniel."Here's looking at you, kid." Atlantic Monthly, February 1975, pp. 39-48, 81-84.
Day, Barry. The Cult Movies: Casablanca." Films and Filming, vol. XX, no. 11, August 1974, pp. 20-24.
Greenberg, Harvey R. "Casablanca. If It's So Schmaltzy, Why Am I Weeping?" in The Movies on Your Mind. Film Classics on the Couch, from Fellini to Frankenstein (New York: Saturday Review Press/E. P. Dutton, 1975), pp. 79-105.
Harmetz, Aljean. Round Up the Usual Suspects: The Making of Casablanca. New York: Hyperion, 1992
Haver, Roland. "Finally, the Truth About Casablanca." American Film, June 1976, vol. I, no. 8, pp. 11-16.
Henreid, Paul with Julius Fast. Ladies Man. An Autobiography. New York: St. Martin's Press, 1984.
Koch, Howard. Casablanca: Script and Legend. 50th Anniversary Edition. London: Aurum Press, 1992.
Lebo, Harlan. Casablanca: Behind the Scenes. New York: Simon & Schuster, 1992.
McArthur, Colin. The Casablanca File. London: Half Brick Images, 1992.
Miller, Frank. Casablanca: As Time Goes By. London: Virgin Books, 1992.
Raskin, Richard. "Casablanca and U.S. Foreign Policy." Film History 4, 2, 1990, pp. 153-164.
Siegel, Jeff. The Casablanca Companion. Dallas: Taylor Publishing Co., 1992.
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2 Harvey R. Greenberg, The Movies on Your Mind. Film Classics on the Couch, from Fellini to Frankenstein (New York: Saturday Review Press/E. P. Dutton, 1975), p. 96.
3 Paul Henreid with Julius Fast, Ladies' Man: An Autobiography (New York: St. Martin's Press, 1984), pp. 120-121. Though Henreid apparently believed he had been promised he would get the girl at the end of Casablanca, his contracts bear no such indication. See Aljean Harmetz, Round Up the Usual Suspects: The Making of Casablanca (New York: Hyperion, 1992), p. 100.
4 Ibid., p. 122.
5 Nathaniel Benchley, "Here's looking at you, kid," Atlantic Monthly, February 1975, p. 44.
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