Wear something nice. It's better to wear something too formal than look too casual. You want to look successful and professional. Remember, if your first impression is bad there will be no second impression. Do not show up wearing a t-shirt and short pants. Wear a suit if you can. There are places around Seoul that rent office suits. For men, wearing a tie with a suit is usually considered optional.
Be neat and clean but don't wear perfume. The scent of your cologne might be offensive to some people.
Be on time. Don't be late to your interview. Being on time means arriving 15 to 30 minutes early. If you are running late, then make a call to apologize and tell them what time you are going to arrive.
Turn off your phone. Right before the interview begins, turn off your phone completely or switch it to airplane mode. Make sure sound is turned off. Make sure you don't turn off your phone too soon so that they won't be able to contact you when you are not there yet.
Practice for the actual interview. Do not walk in without knowing what to say for any question. Make a list of potential questions they might ask you and prepare yourself on how to answer them. Find out whether the interview will be conducted in English or Korean especially if your Korean is bad. If they want to have the interview in Korean, then ask if it's okay to bring someone as a translator.
Give simple and short answers. Do not give a three-minute speech when they are expecting a simple response. Try to find out how long your interview is supposed to be so you can manage your time to your advantage.
Be honest. If you don't have any experience, don't lie and tell them that you do. There's a very good chance that they will catch you lying one way or another. If you lie during your interview and somehow get the job, you can still lose it later if they find out that you made stuff up.
Smile. Show them you've got that positive vibe they are looking for without looking too creepy. There's a saying in Korea; you can't spit on a smiley face.
Turn a negative into a positive. Many Korean interviewers like to ask "what are your strong points and weak points?" Be careful because this might be a trick question. Make sure your weak points are actually strong points in disguise. For example, say the following. "My weakest point is how I try to be a perfectionist all the time. I do not quit unless everything is perfect."
Put on your best behavior during and after the interview. Your interview does not really end until you leave the place. Do not loosen up and end up making a mistake right after you finish the interview.
Assist the interviewers. More often than not, even big companies don't hire professional interviewers. Instead, they have someone at the organization conduct interviews. This means the interviewer(s) might be as nervous as you are during the interview. So take the lead from time to time and be initiative. Ask for their permission to tell them something about you that they forgot to ask.
Make a Korean resume called ee-ryuk-seuh. Korean resumes look more like Excel spreadsheets than Word documents. You would have to attach a photo of yourself at the top left corner on the first page. If you didn't get a specific resume document, then search on the web for a generic form. If they already have your resume, you won't be expected to walk in with a copy.
Do not shake hands unless they reach out to you first. Instead, stand in front of the interviewer(s) and bow your head gently one time. Tell them your name, your age, and where you are from. If they want to shake hands, do not try to crush any bones. In Asia, handshakes are usually very gentle. A little head bowing while shaking hands is considered nice.
Do not look directly into the interviewer's eyes. Instead, stare at right between his eyes and nose. In Korea, looking directly into someone's eyes can be viewed as disrespectful.
Do not carry anything with you. Usually in Korea, interviewees show up without a suitcase or purse. (If you need to have your resume with you, then carry a small binder/folder.) Carrying a purse or handbag for female interviewees is likely to be okay but avoid if possible. (If you are male, just don't.) Leave the purse before going into the interview. If possible, ask someone to hold it for you.
Do not give memorized answers all the time. Show them that you came in prepared to answer their questions. But giving them memorized responses every single time a question is asked will make the interview boring. Just make sure your responses are not too long.
Do not make a joke that the interviewer(s) won't understand. They might not understand your sarcasm. Always be very clear on what you are trying to say. Use simple words. If you want to make a joke to lighten the mood, then make a light joke that everybody will be able to understand. And of course, don't make fun of Koreans. (Even stand-up comedians are reluctant to make fun of Korean people.)
Do not exaggerate yourself. Be careful you are not showboating. In Asian countries, being humble is considered a virtue. Avoid using words like "very" and "best" when describing yourself. Telling them that you are the very best might make them think you are a lunatic.
Do not lie. Nine times out of ten, they will catch you lying right away. Be truthful and do not exaggerate. If your record seems too good to be true, they might ask for verification later on.
Do not trash your previous employer during the interview. Don't tell the interviewer how much you hated your previous job. Focus on the positive things from the past and talk about how disappointed you were when you had to leave the place. This will show that you are a type of a person who loves his job.
Do not ask about what your salary is going to be. In Korea, it's usually a no-no to talk about money during an interview. A lot of Korean employers think it's rude for an interviewee to even bring up the subject. Postpone any questions about how much money you will make if you get the job. Once they tell you that you are hired, then you will get a chance to find out about your salary. It's never too late to walk away from the job if the money isn't right. If they ask you during the interview what you expect your salary to be, tell them you'd rather get the job first and discuss the subject later.