Like the ice melt during February and March, Black Christmas progresses slowly, a downward spiral that takes the entire hour and a half to wrap up. It works in the same vein as the early slashers did - and it never suffers from leaving a plotline behind, since it's unwavering focus on including all characters and all of the minutest details ensures that nothing remains wrapped for Christmas.
Some may feel that Black Christmas' pace is a flaw, that the plodding nature of it and the seemingly inane scenes of carolers, a search party for a missing high-schooler, and an encounter between Clare's father and Mrs. Mac are all unnecessary. There's no faulting them for seeing it this way; as movie viewers in the 21st century, we almost expect scenes to flash before our eyes in dazzling displays of cinematography, moving ever forward in the exposition with only the most cursory of glances towards secondary characters.
But to pass up on Black Christmas as a seasonal treat means missing out on some of the best character development in horror. The glacial pace, like the slow dripping of icicles from the eaves of roofs, is deliberate; that's evident from the camerawork, with slow circular turns replacing quick edits or choppy spins. The camera's exploration not only captures the essence of the holiday - the dimly lit rooms, the colorful displays of lights that give Black Christmas a dark but visually pleasing aura - it also allows director Bob Clark to capture every nuance of a scene.
Since there aren't too many killings within Black Christmas, it's important for Clark to focus on something else. The phone calls from Billy are the most suspenseful, because they are disembodied from a suspect - no one knows where they're coming from, and they're fairly commonplace because they've happened before. There's also the couple of scenes that take place in the attic; it's dimly lit, a stark contrast to the colorful Christmas lights in the sorority house.
But Black Christmas' masterful focus, and the real reason for the slow pace of the film, is the search for motive where there isn't one. The characters are actually important in this movie; unlike other slashers, where teens become fodder for a killer with their main role being pretty pieces of flesh, these people are suspects. Clark treats them as any detective might during a murder investigation: with an unwavering, suspicious eye. Every moment counts, because any one of these people might be the murderer.
Peter, for one, gets a lot of screentime despite being rather bland and uninteresting. He's training to be a concert pianist, and we actually see a scene where he plays a terribly minor, out-of-key piano piece as an audition for some proper, hardened men - it's a trainwreck, of course, but it doesn't seem to fit with the rest of the film, a scene out of place within the context of a sorority's plague of nasty phone calls. But Clark's focus puts Peter at the forefront as a suspect - he's angry about failing his audition, he's not happy with Jess' decision to abort their baby, and he really needs a haircut. For Lt. Fuller, it's a cut-and-dry case of an angry boyfriend. The search for motive leads the viewer directly to Peter.
But it's not Peter - it can't be him. Black Christmas must explore other avenues, and what better place than to focus on another murder within the area during a search party for a missing high school girl. This scene doesn't seem necessary; Clare's father is unlikely to find Clare while traipsing around in the dark, but Clark includes it anyway because it seems to link the sorority house with an external murderer. Maybe there is a serial killer running around town killing innocent girls - despite its vagueness, it's still a motive that the characters and the audience can understand.
And even Mrs. Mac appears, at first, to harbor some ill will for the sorority girls. She's an alcoholic, hiding stashes of liquor in unlikely places, and she's got a sarcasm and penchant for outbursts that make her another suspect for half of the movie. Her encounter with Clare's father furthers this suspicion, and provides a bit of dark comedy that's shadowed by the fact that there's a missing girl somewhere.
Black Christmas recognizes that, at its core, the murders of these women have no explicit motive. Obviously hidden deeply is a disturbing theme of incest only hinted at by Billy's nonsensical gibberish, but for the most part, the film refuses to uncover a motive for the killings. The slow pace, in contrast, is Bob Clark's way of crafting suspense - the viewer expects to find out the reason for the killings, and when there isn't one, it's all the more shocking.
Before the slasher genre became as stale and cookie-cutter as it is today, Black Christmas circumvented the obvious - by slowly pushing the viewer towards multiple suspects, it eludes prediction because this crime cannot be explained, and there is no real motive to make sense of the killings. This warrants the slow pace, and on multiple viewings, it becomes evident that, though there isn't a multitude of events happening at once, there are small layers building to a climax within every scene. The final reveal might be unsatisfying at first, but it's grimly realistic, and the slow ringing of the phone at the end of the film freezes the blood; the icy bleakness of Black Christmas never melts.