Teaching adults is an entirely different ball-game than working with children. Even teachers who have spent years teaching young students often find it difficult to transition to teaching adults, since the class dynamic and expectations are rather different with an adult class.
When it comes to enthusiasm and discipline, adults are by far the easiest group to cope with. Since they usually fork out their own hard earned money to attend your class and attend because they really want to learn English (usually for professional advancement), you'll find them, or most of them, willing participants and not at all resistant and unenthusiastic as many middle and high school students who are forced to study English.
Adults usually WANT to attend your class so disruptive behaviour and disciplines issues will rarely, if ever, by a problem.
However, unlike children, paying adult students will usually not be satisfied with things like games, songs, worksheets and other 'time-fillers' that work just fine with younger students. Adults attend with the purpose of honing and refining their conversational skills and will want as much class time as is possible devoted to speaking and listening activities.
What I personally love most about adult classes is the conversation possibilities: Adults will want to engage in conversation with you, the foreigner. So throwing a topic in the air and simply giving time for spontaneous discussion, where you as the teacher guide and direct the conversation, serves as a great learning tool. I have found that adult students often grab hold of grammar point and new vocabulary in a casual conversation setting, even better than they would in a formal lessons structure. The spontaneity of this method sets students at their ease, drops their guard and makes learning more natural.
This does not mean that you will not need structured lessons, for you surely will. Reading assignments and grammar and vocabulary lessons should always be carefully structured and presented at a level matching the class' ability.
Something to keep in mind: In an honest attempt to add meaningful discussion to a conversation, foreign language students can often take up too much time trying to get a particular point across, or add unnecessary detail which makes their stories too long and boring. This will drain class dynamic. As the instructor your duty is to maintain energy levels in the class by telling students to keep in short and to the point when you see they're starting to drone.
A similar problem arises from adult students with stronger English ability often dominating the conversation and robbing others of opportunity to share. Most often they don't do this purposely, but simply out of excitement! Again, as instructor, the authority to manage class dynamic, is yours.
On the other hand, some adult students will be very shy and unwilling to speak at all. Despite the fact that they know they need to learn English and that they've paid their own tuition, the idea of losing face in front of other Koreans, by making grammar or pronunciation mistakes, is just too horrible for them to comprehend.
I've taught adult classes where certain students sit through entire periods of conversation without saying a single word and when it's their turn to speak they'll just mumble something in Korean to someone else. Which I have to gather means "Pass"! Students like these often thaw with time and begin to engage more freely as class progresses.