This is specifically for Maggie and others who haven't been in Korea in recent times. I learned this the hard way and I thought they should know.
The customary thing is to greet and bow, and exchange short friendly chat. Stand up when someone arrives, like at the dinner or even lunch. Just seating there isn't too cool.
Another is Korean people rarely say 'excuse me' when they bump into you. Don't take it personally.
When elevator door opens everyone rushes in and not worry about who's inside. Be quick and get out of the elevator as fast as possible as they are hyper impatient. This goes for any lunch rooms or commissaries. People have actually pushed me and pushed my tray along with their tray or body. This goes for subways too or anywhere there are crowds.
Spitting and smoking in the street. Guys will usually spit on the sidewalk - don't be shocked, girls will do it too if they've been drinking. There are many smokers in Korea - although, they've implement non-smoking section at Starbucks or Coffee Bean. But, in the street get ready to smell like ash-tray by the time you come home.
Drinking: This is a customary thing - most people will push you to drink, discreetly decline if you can if you think you'd too much. (Say you're taking medicine - and they'll usually leave you alone).
Guys intentionally making eye contact - this used to bother me but don't take it too personally. Just smile back and move on.
I may have forgotten others, but for now this will ease the pain-in-the-arse experience you may face once you set foot in Seoul.