After a minute or so of this, he suddenly strikes a pose, turns this way and that, clenches his fists and ripples his muscles in classic Charles Atlas style. He regards himself over his own shoulder - a show-off session in the privacy of his room.
Or so he thinks. But I'm sitting across the road dead level with him (two floors up, babysitting for our friends a few doors down. Their sitting- room is an airy, slope-ceilinged loft on the top floor).
So far, I've been discreet. I've drawn the blind on one of the two windows so as not to feel too exposed myself; the other I've left up - unwilling to surrender the sun-washed purple and tangerine sky, at 10 o'clock still alight with heat. I'm among the batik cushions on the sofa, positioned somewhere between the two windows. I can just see out, I don't know if he'd see me - if he tore himself away from his reflection.
I'll admit it's something of a thrill, peeking at a man performing, oblivious, to a mirror. We women pout, pull in our stomachs, flit secretly from mirror to mirror, glance furtively in darkly reflective shop windows. But men - oh, joy - flex their biceps, go "grrr", pretend they're toga'd in animal skin.
I'm aware that I'm blessed - this is a rare sighting, an enlightening treat, and it's just come right. I've finished the book I brought, I've found a copy of Harpers & Queen in the loo and read about bowel-cleansing and navy blue, and I've zapped through the channels and watched part of a made-for-TV movie about a man whose wife has hired a hit-man to kill his lover.
Just as I'm on the verge of submitting to the unreality of the ghastly, tabloid Ten O'Clock News, this vivid alternative show begins.
So far, I've only known that house from street level. Our own upper windows give on to muted lighting and Austrian blinds. But these, a few doors on, are rented rooms and I've seen a variety of people come and go with Asda bags and takeaways. One night, I was kept awake until 2am by the sound of hammering. Now I'm interested to see that there is indeed a kind of wooden structure in the centre of the room. Doorless wardrobe? Hangman's gibbet? S & M prop?
The man suddenly flops down from view, and the light goes out, except for the bluish flicker of a TV. Oh.
But no, the light's on and he's up again, still stripped to the waist, placing several pairs of trainers out on the window sill - a habit reminiscent of Australian students in the Italian hostels of my youth.
And then he spots me. Or, at least, he freezes with his face towards mine, towards my smug, snug, halogen-lit look-out post. He stands there, hands on towelled hips, gazing towards me. Has he seen me looking?
I keep my eyes on the TV, suddenly shamed, exposed. Then I have the most marvellous brainwave. I turn to the other part of the room, the part where the blind's drawn, the area he cannot see, and I laugh. I laugh for a bit and then I open my mouth and talk. Then I listen. Then I laugh again, wildly gesticulating. In other words, I pretend I'm not alone. Because if you're in company and you happen to glance up and see someone in the window opposite, then that's all it is. But if you're alone and you're watching them, you're just plain sad.
I keep up this pitiful charade while he stands there, and finally he goes back to his TV. Then I crawl over, below windowsill level, and draw the blind.
Much, much later, back at my house, in bed, I can't sleep. It's a thick, hot night, the pub has long ago emptied, the sky has darkened to aubergine. Everyone's asleep, mouths open, limbs flung out, rucked sheets. I switch on the light, read a few pages of Alison Lurie, switch it off, close my eyes.
At 3am, I'm almost falling, when a noise pulls me back. It's unlike anything I've ever heard: neither quite machine nor animal nor human - something, in fact, between a flooded engine, a bark and a fretting child. It continues for a few minutes before I recognise that I'm awake and it's in the street.
At the first window, I see nothing, but the noise continues plaintively, so I move over and lift the other blind. A huge fox is standing there, in the cool moonlight, in the middle of our street. Head erect and proud, he stares at the distance, uttering his eerie, distressing bark again and again, insistent, implacable. His coat is blue with dark, his feet firm and still on the purple asphalt, his head whipping round now and then.
I watch him for what seems a very long time - just him and me in the middle of the night, fellow creatures on a dark and lonely planet.
And just when I think I can watch him no longer, when nothing else is happening and the moment is frozen forever in my head, there's a flick of movement at a high window and there's my mirror-man, forearms on the sill, watching me, watching the fox, each one of us apparently - mistakenly - alone.