This is so ADVICE from past and current JET's in Oita prefecture. Please remember, everyone is different and this advice may or may not apply to you.
Interacting with Co-Workers
Initially be polite to your teachers (most will soften up pretty quickly)
Don't surprise them (give them notice of you have an idea)
Be flexible, every JTE wants something different
Be confident and a little steadfast when it comes to responsibilities as some teachers will happily push all responsibility to you. Being a 'little' stubborn/difficult can be a good thing.
Usually young teachers are very good for making friends. if they're English teachers, they'll be a good bridge to the young non English teachers
Communicate with your JTEs before lessons to plan and schedule. Open communication is different depending on teacher. Some like emails, others like post it notes, and some need one on one meetings.
Show up on time (very important in Japan)
Be flexible and understanding
Your attitude determines how your coworkers will think of you, especially if you don't speak Japanese fluently. Smile and apologize like a Canadian.
Learn your introductions
If you are placed in elementary school I strongly suggest learning as much classroom Japanese as you can before you start.
If you have a lot of repetitive lesson plans then vary it up a little (for your own sanity)
Have a couple of go-to games as teachers will often ask you for some out of the blue (ask any current ALTs and they always have a few to share)
Talk to other ALTs about their lessons, never be afraid to ask questions (whether it's in school or to fellow ALTs, everyone gets totally lost at some stage)
You may feel like you're not being used 100% of the time to best of your ability. But that's not to say that you can't change that and meet halfway. Try to insert stories, expressions, phrases and anything else you can in junior high classes. They will appreciate your English, your interests, and tips. Try to be as proactive as you can (i.e. walk around the classroom and check students' spelling, handwriting) if you feel that you're not being used at all in the classroom.
Be full of games/activities
Always have a lesson in your back pocket that requires no prep. IE: Tongue twister games, pictionary, etc.
Be mentally prepared to teach alone and happily surprised to team teach. Try hopelessly to ask teachers for lesson topics, then have ideas to propose. ALWAYS have ideas to propose (horoscopes, travel lesson, holidays, ). They want to be a tape recorder? Fine, ASK for other responsibilities, can I grade writing assignments? Can I look at their exams? Do you have an extra copy of the textbook (secretly to base lessons on). Also, even if its after working time, stay for cleaning time. The kids love it, you"ll learn to love it, and damn, no one cleans the staff kitchen. Get on that.
Interacting with Students
Have energy with the students (making English fun is one the main roles of an ALT)- chat to students in the corridor, play outside (just the odd time)
Your capacity in this job is limited, so just relax and try to make the kids smile (they're stressed)
Relax, have fun, don't get discouraged if students aren't interested.
Students are watching you ALL THE TIME so be aware of your actions.
Most importantly: Don't worry about it, you're a foreigner, you WILL screw up some things but they'll completely understand!
Don't be afraid if you have a lot of schools. You role as an elementary English teacher is to visit once a week and make English class exciting and fun. You whoosh in and whoosh out, and there are positives and negatives to that.
Make the best of it. Try to talk to people and be friendly
My main tip is don't work yourself up about it. To start with I was super nervous but really it's fine.