For the past month and a half, I've conducted five private lessons with a non-Japanese Asian woman. This wasn't my first experience teaching a non-Japanese person, though I can say that such experiences are not all that common. Because of their relative rarity, they stand out in stark contrast to what it is like to teach Japanese people and often make me consider how teaching English well to one group of people doesn't necessarily prepare you to do so for other people.
From my past experiences with teaching Chinese folks, I knew that at least two things were going to differ compared to my usual lessons. First of all, she was almost certainly going to be more assertive in lessons and require fewer leading questioning to elicit her opinions. While not all Japanese people express themselves reluctantly or are passive, Tokyoites are renowned for their reserve and, by and large, this is reflected in their lessons. Second, her pronunciation was likely going to be harder for me to understand since the types of mistakes made by Chinese speakers of English are not like those made by Japanese speakers.
When I have a non-Japanese student, it brings home rather strikingly how experience makes you a better teacher mainly for the type of people you typically encounter, but not so much for everyone in general. The fact is that you come to understand what your students are saying better as you become more familiar with the types of problems they typically have. There are words that you would never have recognized or understood if you had encountered a random Japanese tourist back home which you have no problems whatsoever understanding after spending some time teaching.
While this recognition is a blessing in terms of understanding what your students mean to say, it can end up being a curse in the long run. If that recognition means you stop correcting or noticing the mistakes because you've grown so accustomed to them that they no longer hinder your comprehension, you can end up not helping your student improve in ways that will assist them in the real world of English communication.
Another problem that comes along with experience is understanding and giving into the impulse to speak Japanese when you're an English teacher. Teachers who are even roughly proficient in Japanese often end up translating for their students rather than conducting a lesson only in English. Typically, a student will mumble the word in Japanese that he can't come up with in English and the teacher who recognizes the word will confidently and quickly offer up the English. What is worse though are the teachers who fail to explain things in English at all in a simple and slow fashion and ramble on in Japanese. The problem with this sort of teaching is that it doesn't force the student to think in English or learn to work with the vocabulary he has. It's very important for people to learn to be able to say what they have to say indirectly when they can't do so directly as it will serve them well in travel and business situations which require English. For instance, if the student can't say "thief", he can at least say the equivalent, "a bad person who takes things things from other people to get money."
Instant translations decrease the likelihood that the student will remember the English as he made no effort to find the meaning of a word and will grow accustomed to not having to study on his own with a handy living gaijin dictionary in front of him. That's not to say there isn't a time and a place for teaching English basics in Japanese but rather that that time is almost never when a native speaker is doing the job. It's a case for Japanese teachers who have training, qualifications and skill in teaching low level students with no or very rudimentary skills.
In both the case of the teacher who knows Japanese-English well and fails to correct mistakes and the teacher who uses Japanese, the student is being done a disservice if the study is about actually improving the students' skills (rather than about a paid, structured social interaction with a foreigner). Those teachers who use Japanese generally do so to make their lives easier rather than to help the student and there's really no excuse for it because it's easy to avoid using Japanese a lesson. With omnipresent electronic dictionaries, teachers absolutely do not need to translate for students. Even the act of punching in a word and reading what is on the screen for themselves will provide a small increase in the chance that a student will remember the translation compared to a teacher blurting out the word.
The other case involving a lack of correction due to so much familiarity with Japanese pronunciation and English errors is one which I find myself having to be vigilant about because it's not as cut and dry as choosing not to speak Japanese. I don't correct every one of those types of mistakes because sometimes it'd bog the lesson down completely but I do try to focus on certain sounds which are problems and correct them assiduously for a particular student until there is some improvement before moving on to the next one. Still, I'm not certain that I'm not doing as well with this as I might if I weren't so experienced.
It's a bit ironic that experience and cultural understanding may actually contribute to one being less effective in some ways when one is a teacher. Certainly there are ways in which having experience really helps. You learn the roots of typical grammar problems because of translation errors and the incomprehensible can be recognized and corrected more efficiently. Additionally, students have greater confidence when they can make themselves understood well and a lack of confidence is one of the bigger issues with Japanese people speaking English. Pacing lessons also becomes second nature as is how quickly or slowly to speak based on a student's level. All that being said, those that are truly interested in helping their students have to keep their impulses to make things easy in check.