Once these initial thoughts have flown disruptively through your mind, you are able to get back into the zone. You're concentrating again; your nerves are soaring a little higher than before, but you enjoy that feeling. Then comes the real question: what is the etiquette now?
A moment of eye contact and a slight change of expression, if the show isn't too serious – this is surely acceptable. Nobody else would notice – again, as long as the show is of a certain nature. For example, you could probably get away with smiling towards your Familiar Face in Mamma Mia! – you're smiling most of the time anyway – but maybe not so much in Les Misérables.
The interval can be a time of temptation. Your Familiar Face is just outside the door; couldn't you just poke your head round and say hi? The answer is always no – resist. That is not to say that all interval communication is entirely off limits – why not send your Familiar Face a quick text, if you have time? The ideal compromise.
During the bows you can almost always get away with glancing at your Familiar Face, and giving them a sort of 'I did it!!' grin. Hopefully they'll grin back – if they don't, just carry on smiling as though you had intended it for the whole audience (and try not to immediately jump to the conclusion that 'they hated the show'). They were probably still spellbound by the epic production – tell yourself that.
After the show, it is perfectly acceptable to throw your costume off in a fervour of impatience, rub the make-up off your face so vigorously that is stings a little afterwards, chuck on normal clothes and rush out of the dressing-room looking like you've just been in some sort of accident. As long as you're not in costume or make-up, there's nothing stopping you from literally running into the arms of your Familiar Face. However, you may prefer to come across as slightly more elegant – in which case, walk.
Well, that's what to do if you're performing and you see a Familiar Face – but what if you're in the audience, and your Familiar Face is in fact on stage? The etiquette here is much simpler: leave it up to the performer. If they smile at you, smile back. If they text you in the interval, text back. If they literally run towards your open arms after the show, don't side-step.
Oh and, of course, you have to tell them the show was fantastic, whether you enjoyed it or not. There are some fundamental aspects of Theatre Etiquette that must always be followed.