Photo: Cornelia Schauermann/Getty Images

I had to mumble “sorry” to a woman sitting next to me on the subway today. Twice. She was meticulously applying eyeliner and mascara — and I was completely mesmerized. The way she timed and calibrated each stroke with the hurtle of the train. Her complete concentration and surgically steady hand. The sheer balls it takes to do three coats of carbon-black mascara while going 45 mph at 7 a.m.

My furtive glances must not have been all that furtive, though, because in between coats, she shot me a withering look that made it clear I was breaking the unwritten rule of mass transit: Feign indifference at all times.

But I am not indifferent. The current subway PSAs place public grooming firmly in the no-no category, and I’ll probably be in the minority when I say this, but I like to watch. I really like it. I actually will switch places to be closer to someone doing their full face in the same car as me.

I’m not looking for makeup tips. I’ve actually spent many, many years writing and editing all about beauty, so even though I am rather clumsy with my own makeup skills, I know far more than I care to admit about shading, shaping, and contouring. And I’m not watching because I’m grossed out (unless there’s nail clipping, which is unacceptable). Nor am I watching because I’m simply bored.

Much in the same way some people find eavesdropping on a sparring couple at a restaurant a moth-to-a-flame guilty pleasure, when I get a front-row seat to someone else’s beauty routine I’m similarly riveted; I stop whatever I am doing to take it all in.

And finally, after today, I think I’ve figured out why.

I wasn’t in a sorority. I don’t have sisters. During those years when everyone was “pregaming” at someone else’s house — angling for the mirror and sharing eye-shadow quads while tossing back cheap liquor — I was … I don’t quite know where I was. Part of the time, in college, I was busy being a jock, so I guess I was working out or at practice. Part of the time, I simply wasn’t doing all that much put-your-face-on partying, so there wasn’t much prep to be had. I was also not much into group living: During four years of college, I managed to have roommates for just one and a half years. After graduation, I quickly started living alone.

The end result: I’ve never spent much time with other women while they get on their game faces. Sure, I see the finished product everywhere, but rarely do I have the chance to go behind the curtain, to see how the sausage is made. To know what those crazy-long lashes looked like before the five crazy-aggressive flicks of the wand; to see the eyelids before the cat-eye gave its first purr.

And while I can quickly conjure up a smokey eye in my mind’s eye, I’m amazed by all of the ways people actually get there — whether they have a different brush for every layer; whether they’re creating it all with the el cheapo stuff from the drugstore or springing for the fanciest brands. I like watching women who clearly have a very practiced routine, barely pausing from one step to the next, fingers fishing through their pouch for the next item without even glancing down. I get a kick out of seeing if they are following all of the various tips and tricks doled out in women’s magazines or if they are going rogue.

And I wonder: Is this an everyday thing or a running-late exception? On days when I don’t have time to do any makeup, I just go without — or maybe swipe on more lipstick. Honestly, I’m impressed by the commitment to beauty that’s demonstrated by women who whip out their bag full of tricks on the Q train.

But it’s more than that, I think. It’s not simply curiosity about the intricacies of technique or my fundamental nosiness about why someone has eight different kinds of makeup brushes in her purse. In fact, I’d argue that I’m also compelled to stare out of a bit of wistfulness for a kind of girlfriendlike camaraderie I miss, especially because it was so truncated in my life.

As my friends and I have gotten older and disappeared into the demands of our adult lives and our families, we’ve lost those shared moments of communal prepping — be it in the kitchen, the bathroom vanity, or anywhere else — that can be the steady undercurrent to intimate female friendship. I know it may seem like a stretch to say that I find a bit of sisterhood (as one-way as it may seem) in watching another woman do her makeup, but I do.

So, to any subway sister who catches me shamelessly staring while you line your lips, consider this: Sometimes the most beautiful part is not the before or the after. It’s the part in between.