It's easy to understand why you'd compare them. Both pictures are about the making of a movie. In each, there's a beautiful lead actress who gets a lot of attention and is something of an enigma. Actually, that's where the differences start. In Day for Night, the arrival in Nice of British star Julie (Jacqueline Bissett), to shoot the film-within-a-film Meet Pamela, is a major press event; the flash bulbs start going off, as it were, in a series of white flash-frames, before Julie's even off the plane. The arrival of Hong Kong actress Maggie Cheung, playing herself, at the Paris office of Niama Films, is greeted with confusion and mild annoyance, even, by sundry members of the production crew, who've got other things to do. Cheung, as it happens, is the pet obsession of Vep's film-within-a-film director Rene Vidal (Jean-Pierre Leaud, the Truffaut discovery who portrays an actor in Day). To everyone else, at least for the moment, she's a bit of novelty.
In Day, Julie soon settles in as an important but not overbearing member of a small and fraught fraternity, as it were, while in Vep Maggie remains at a remove even as she has to respond to the attentions/demands of Vidal (who, contrary to the surmises of several critic, is never actually referred to here as a former "Nouvelle Vague" director) as well as the awkward, passive/aggressive romantic interest of costume designer Zoe (Nathalie Richard).
This brings us to the basic thematic difference between Day For Night and Irma Vep—Truffaut's film is largely about moviemaking as a microcosm for life itself, while Vep is largely about the cinema (and the cinema icon) as a cynosure of desire. "Desire," Vidal tries to articulate to Maggie, is what's it'a all about; and near the end of the film, when Vidal's proposed remake of Les Vampires with Cheung in the legendary role of Irma Vep has all but collapsed, Maggie tells Zoe she "understands" Vidal. The desire that Vep is about also lies outside its diegesis: Assayas married Cheung in 1998. (They divorced in 2001.) In a letter Jean-Luc Godard wrote to Francois Truffaut in 1973—the communication that precipitated the final rupture between the one-time friends—Godard says Day For Night is a lie, and Truffaut is a liar, "because the shot of you and Jacqueline Bissett the other evening at [restaurant] Chez Francis is not in your film, and one can't help wondering why the director is the only one who doesn't screw in [Day For Night]." A ridiculous implied accusation, to be sure, but it does say something pertinent about the undercurrents of a film, particularly the undercurrents of a film about filmmaking.
Day For Night's tone is altogether lighter, friendlier than Vep's; Assayas' depiction of the speed and disconnection of the "real" world surrounding the crew of Vep's film-within-a-film can indeed be frightening, but it's also sardonic, sometimes despairing, angry. (It's a tone he'll distill further for the very potent brew that is demonlover.) Assayas has cited a much darker moviemaking movie than Day For Night as a strong influence on Vep; that is, Rainer Werner Fassbinder's acidly funny 1971 Beware of a Holy Whore. It is perhaps no accident that near the end of the film, Whore's Lou Castel—himself, like Leaud, an icon of a particular bygone era of cinema—turns up as a director even more seemingly dissolute than Vidal. One who, nonetheless, will be taking over Les Vampires, without Cheung.
Castel in Beware of a Holy Whore
Castel in Irma Vep
The characters played by Leaud and Castel never confront each other; three years prior to Vep, though, the actors played best friends in a film by another Assayas hero: Philippe Garrel's The Birth of Love, itself a quite remarkable film. Threading through all the divergences and correspondences I've cited: fierce intelligence, fierce passion, fierce love/hate of cinema. For all their differences, Day For Night and Irma Vep make an exemplary double bill. Hell, add Whore and Love (if you can find the latter) and you've got a great quadruple bill....