I think it depends on the person, but when it happened to me, I was very quiet, in my own world a lot of the time, I had no inclination to do anything, I didn't want to be left on my own, I was very tearful, and lethargic, had to practically be dragged out of bed, and made to eat too, to be frank I just didn't care about anything.
If you think someone close to you is having a breakdown, then I suggest they get to their Doctors asap, and all you can really do is be as supportive as you can towards them. my parents got me a kitten, and she was a godsend, she always gave me hugs and fuss when I felt at my worst, and she helped me a great deal.
Believe me if it happens, you'll know it.
Lack of concentration, suicidal thoughts, constant crying and a total change of usual character. it is not nice when this happens to someone. A friend of mine suffered and took her own life. So if you do think this is happening het help or offer help asap.
If some-one you know is acting out of the ordinary and keeps getting worse then may-be. they could just be really stressed. if your worried maybe get intouch with family? A nervous B D can gradually build up till the person can't tell what's "real" or not..