Sunday Bloody Sunday is one of those films that lets you into the lives of believable, complicated characters. A handsome, self-centered young artist played by the actor/rock singer Murray Head is having simultaneous affairs with both an older woman (played with infinitely nuanced self-irony by Glenda Jackson) and an older man, a Jewish doctor (the touching Peter Finch), two intelligent adults who have mutual friends and even know each other slightly. Each of them is aware of his or her rival and accepts the necessity of sharing the young man, who seems to love them both, though neither is as important to him as they would like. The characters are equally unsentimental and realistic about their possibilities for happiness.
One of the elements of the film I most admire is director John Schlesinger's use of the sublime trio from Mozart's opera Cosí fan tutte as a running theme. The untranslatable title of the opera means something like, "They're all like that" or "That's what they all do." The doctor loves opera, and this is the music he plays for his lover and what he listens to when he's alone, waiting for him. But the music keeps recurring on the soundtrack as if it's in the heads of all the characters.
It's a trio of farewell. In the opera, two sisters think their lovers have been drafted. The two girls and their lovers' older friend are praying for smooth winds and calm seas. "May every element respond benignly to our desire." But Mozart interjects a disturbing harmony on the word desir — desire. Do these young women know what their real desires are? Do any of us?
The harmonic twist on the word for desire brings to the surface a twist of Mozart's plot. We know something the two sisters don't: Their lovers are only pretending to leave; their older friend has bet them that the girls won't be faithful. The boys have agreed to return in disguise, and each attempts to seduce the other's fiancee — and they succeed. Love is more complicated than any of them thought. At the end of the opera, Mozart and his devious librettist, Lorenzo da Ponte, leave it up in the air about which couples will pair off. And the older man who perpetrates the bet doesn't seem any happier for winning it. This music expresses perfectly the poignant emotional uncertainties of Sunday Bloody Sunday.
Here's what Schlesinger himself has to say about his choice of music: "First of all, I like music very much. It's very much part of my life ... I know I wanted to use that trio from Mozart. It was just a lovely sound. It's simply a piece of music which I just liked, and that was the only reason I really used it, I suppose."
I'd say that if anything, Schlesinger was understating the importance of this music to his film, and that he intuited some very deep connection. Years later, Mike Nichols chose the same music for his film Closer, another film about couples changing partners. I'll bet anything that it was his homage to Sunday Bloody Sunday.