Teaching English in Japan requires reaching a wide variety of students. There are students who lived in America for years and are essentially fluent. There are students sleeping at their desks. Almost fear-inspiring energetic elementary schoolers, painfully hip high schoolers, and... adults.
Teaching English to adults in Japan have been some of my best lessons in Japan, but some of them have also been pretty uncomfortable. Below are some points you should keep in mind when planning and conducting your adult lessons. Hopefully you'll enjoy your adult lessons too, or at least avoid abject failure.
Most classes taught to adults are heavily focused on spoken English. The Japanese term used for these classes is eikaiwa (literally, English Conversation). Thus the awe-inspiring "read the reading", "recite the reading", "read the reading after me", "read the reading after your partner", "read the reading after your partner facing the left wall", "read the readin..." style often found in junior high schools will invoke a student revolt. These classes are focused on providing practice listening and speaking, and thus the students are intent on having at least some degree of dynamicism in the lessons. They will feel cheated if they don't have opportunities to speak. (Corollary: They will feel hunted if they have to speak too much.)
Range of Ability
The range of ability in English classrooms is always an issue, but it can be even more of an issue in adult classes. Some adults want to learn English, but will enter your class with essentially no knowledge. Their classmates may be somewhat conversational. Some of the students may be fully conversational. Others may remember a bit from their school lessons ten or twenty years ago. Teaching a lesson that is accessible to all of these students can be very difficult.
There are a handful of was to address this reality. First, practice should always involve activities that allow the student to play to their own level. For example a self-introduction might be three or four sentences for a beginning student, but could be much more complex for an advanced one. Another recommendation is to pair students of equal levels together. This doesn't allow more advanced students to help the less advanced ones, but it goes a long way in keeping everyone challenged, interested, and happy (aside: unhappy adults are very unhappy).
Large Vocabulary, Unpredictable Grammar
My experience is that you should use activities that allow the use of lots of vocabulary. Many adults will have acquired a fairly large--and completely unpredictable--vocabulary in English. On the other hand, they may not know or remember any English grammar. As such, I have found that teaching phrases has been very successful. The more advanced students will be able to break apart the phrases into their components, but the less advanced ones won't be able to construct a coherent phrase from its components, so its important to give them something real and complete that they can then customize.
Adult Sense of Humor
Contrary to what some people proclaim, the Japanese do have a sense of humor. When dealing with younger students, though, it can be near impossible to figure out what that sense of humor actually is. Other than abject failure or humiliation, I don't really know what my junior high or elementary students find funny. But, the adult students in my conversation classes, I do know what they found funny. Because it was the same stuff that I find funny. Using your sense of humor can really help when teaching to adults. A comfortable classroom is a classroom where people can learn. Humor is a great tool for achieving that.
Also, adults are going to be interested in many of the same things that you are interested in. Politics, cars, their families, jobs, other countries. These are all topics that will resonate with your adult students. This is a fun opportunity to talk about topics you are more interested than the latest manga or television drama.
Games? No, no. Learning is Boring
By the time you've made it through a year of Junior High School in Japan, you have learned one truth: learning is pain. The adults you are teaching are intimately aware of this. Does this mean you can't do games? No. Just don't call them games, call them activities. This is a fundamental truth I wish teachers I team-taught with would understand: when you call something a game, then everything goes to hell. Just call it an activity, and if it happens to be a fun activity, then people will have fun.
However, as a caveat, these are adults, and some won't be very relaxed or comfortable. Games that require a lively and energetic crowd may go over poorly. At some point it becomes embarrassing for many adults to be competitive about things that don't 'matter'. Keep this in mind when creating activities.
Teaching in Japanese or English
When teaching long term classes you have the time to build a foundation with your class in English. The students get used to your classroom instructions, "Stand up", "Sit down", "Make pairs", and you can gradually conduct much of your class in English. Its hard to build that environment in a class that meets less frequently, or has fewer total lessons. As such, and depending on your long term goals with the specific course you are teaching, you may find that the benefit you get from using exclusively English is quite low.
Adults--just like small children--get frustrated, bored, and distractable if the course is too difficult. Its helpful for them to have some periods during the class that are relatively low stress. Explaining some parts of your lesson in Japanese can help create periods where your students can regroup and regain focus if your English has been difficult for them to understand. In particular, explanations of grammar can often be done more easily in Japanese.
Topics to Consider Teaching
Below are some topics I have tried teaching in my adult conversation classes. Almost any grammar point from a junior high school text could be taught with good results. In addition, looking through an introductory or intermediate Japanese textbook was a good source of ideas for me.
Have you ever ~ ?
must, and musn't
have to, and don't have to
should, and shouldn't
prepositions (in, at, on, outside, inside, left of, right of, near, above, below)
proverbs ("The tortoise and the hare" is a good starting point, since they have an almost identical story in Japanese)
I can, and I can't
forming and using noun clauses
asking and answering who/what/where/when/why/how questions (broken into managable chunks)
Its near impossible to have reliable expectations when walking into an adult conversation class. Your students may be extremely talented, or merely humoring an old dream of theirs they haven't made any substantial effort to fulfill. The first day you will walk in, and you will have to start improvising. That improvisation won't stop until your lessons do. Which, as it happens, is why adult conversation classes can be so damn enjoyable.
Let me know if there is any feedback, have any questions, or any complaints!