In a sense, almost all lovers role play, in the sense that all artists lie, or at least create artifice with consent of the audience. There's a balance to maintain between the delightful illusion and the comfort taken in knowing someone accepts your imperfect, human reality. A balance normally sought by two, but what if there are three as in 1962's Jules and Jim? A beautiful, seminal work of the French New Wave by Francois Truffaut it achieves an incisive perspective on the nature of heterosexual relationships in its challenge to conventional portrayals of romance in film.
If there are three people, why does the title only refer to two? If it's heterosexual, why are they both men? Why is actress Jeanne Moreau on most of the posters and box art? These questions lead us directly to the heart of the film. Catherine, Moreau's character, may be seen as a perfect reflection or expression of the two men. This is a sort of reality, a working dynamic, but it's flawed because it denies Catherine participation in the creative process. Functionally, Catherine does contribute.
A recurrent image is of Jim (Henri Serre) and Jules (Oskar Werner) playing Dominoes and Catherine sitting discontent apart from them, unable to participate. Before they even meet or hear of Catherine, the two men encounter a statue that resembles her that they both agree meets an aesthetic ideal, and the two men are intimately united in their aesthetic ideals. They refer to rumours that they're lovers but they don't sleep together; they merely share an intense pleasure in knowing someone who understands so perfectly--they endlessly discuss their philosophies of aesthetics, two auteurs improbably united in vision.
One could look at them as film directors--possibly even as Truffaut and Godard--and Catherine as their leading lady. But the reality of meeting the statue immediately meets with an instinctive discontent from their object. On their first outing, Catherine makes the apparently unplanned decision to disguise herself as a man, perhaps unconscious herself of her motives.
Immediately she wants to break with their ownership of the ideal she embodies and yet, just like New Wave directors, both men love it. They love the subversion of their control which paradoxically fits into an even better picture of their ideal. So Catherine feels validated in attaining a role in the creative process though she's far more instinctive about it than the men, Jules in particular.
While Jim is famously great with the ladies, Jules refers vaguely to a girlfriend he had back home in Austria in a way that suggests he's never actually been with a woman. Before they meet Catherine, they run into a woman named Therese (Marie Dubois) who's obviously very open to casual sex so Jim encourages Jules to take her home with him. It ends up as a sexless sleepover, Jules not even managing to interpret, or subconsciously choosing to misinterpret, Therese's "locomotive trick" with her cigarette.
But it's Jules that Catherine ends up with first, largely because Jules pleads with Jim beforehand; "Not this one." After the two have been married and have a child, after World War I, Catherine confides to Jim she thought she'd eventually crack Jules' shyness but finally realised that it's simply part of him. Instead of "merging" she says they ended up coming "face to face". The tragic aspect of this is that they love each other and each loves the incompatible quality of the other.
After the three of them have seen a play that Jules and Jim have both hated and Catherine loved, Catherine talks about how she loved the way the woman in the play "invents herself". This leads Jules to go on a rant and deliver a long quote from Baudelaire about how contemptibly "natural" women are. At which point Catherine throws herself in the river, something which the film's narrator describes as inspiring deep admiration in Jules, causing him to imagine himself going into the river with her.
One is strongly reminded of Alfred Hitchcock's Vertigo which also features a woman throwing herself into a body of water and is also about a woman's attempt to satisfy her own assertion of identity while embodying the ideal of two men. Of course, the influence of Hitchcock on the New Wave filmmakers is well documented by the filmmakers themselves as Truffaut's book length interview with Hitchcock attests.
There's also a Christ-like quality to Catherine's sudden sacrifice in the face of Jules' contempt for the "naturalness" of women--she dies for his repulsion which makes him see this as a negative quality in himself. He imagines himself in the water with her because what she did is seen by him as a better expression of himself. Once again, she embodies the ideal of the men even in an act that would seem to be purely independent. In a structure of relationships that fundamentally puts men in control and women as the stage or product of an artistic endeavour, she in some sense is male. This comes across not only in the cross dressing scene but in a song she writes and performs later in the film, the lyrics to which are entirely devoted to admiration of female beauty. Of course this suggests there's a flaw in seeing one role as intrinsically male or female.
The song's title translates as "Whirlpool" which recalls the spiral visual motif of Vertigo as a metaphor for the observer being pulled in by obsession.
Jim, meanwhile, is compatible with Catherine in the way Jules isn't. She didn't need to jump in the river for him, he agreed with her that Jules was behaving badly. As mentioned earlier, Catherine confides in him, tells him her motives for cheating on Jules as best as she understands them, as revenges for his insensitivity. Jules confides in Jim that he knows her sleeping around is done out of revenge but has no idea what he's done to provoke her.
Jim describes Catherine as only having appeal for him when they're alone together, in other words when she is in confessional mode instead of performance mode. This in some sense merges Jim and Catherine in the way she complains of not being able to do with Jules. But Jim and Jules still share their symbiosis of aesthetic philosophy while both Jim and Catherine pity and admire Jules' insensitivity to passion. The obsession spiral is perfectly realised in this three endlessly seeking satisfaction from each other while accruing more and more pain with each revolution.