You have two seconds to assess the situation and decide your plan of attack. They have an outstretched arm. You prepare for arm-on-back contact. But what’s this? Face! Face is heading towards yours at an alarming rate. Where is it going? Shoulder? Ear? Cheek? Left or right? QUICK, manouevre appropriately. BAM, you’ve got lip-on-cheek action. Ok. A kiss.
But while the residue is still wet on your face, you’ve got another judgement call to make. Are they going in for the other? Can you pull away or are they cannoning off to the other side of your head like a puckered guided missile? Is there any danger of nose-brushing? Or worse, have you gone in for a second cheek while they’ve started to pull away? Are you doing the dreaded this-way-that-way face dance? Are you wishing you’d never turned up?
Of all the social minefields we gingerly traverse in every day life, this is a pretty old one. I reckon there were Medieval serfs groaning into glasses of mead, going, "and then I thought she were goeth in for other cheek and I landeth smacketh on her lips." In more formal times the handshake was king - what a man does standing up, a lady does sitting down and a dog does on three legs, as the joke goes - but in our touchy-feely-gropey age, even with perfect strangers you can expect anything on a scale from curt nod to tongues.
Meaning no offence to our continental cousins, Brits doing the two-cheek thing just looks affected. It screams, 'I saw this once in a film'. We don't do it well. We dither, we hesitate, and can't avoid adding an onomatopoeic flourish every time. You don't have to SAY 'mwah', you just DO a 'mwah' - it's a concept we've still not grasped.
But whatever the outcome of the greeting and however horrifying the mishaps on getting there, one rule stands firm. You must never acknowledge awkwardness. For to cheerily say, "oop, you were going in for another one there, weren't you?" is to destroy your companion with the Sword of Social Discomfort. You have knocked them down when they're most vulnerable. They may not recover confidence for several weeks, reliving the horror every time they encounter a human.
No, you must make the most of whatever dithering, air-smacking, ear, nuzzling, back-patting routine you manage to improvise between the two of you, then swiftly move on. You can always send them flowers later to apologise, if necessary.