Originally Posted by jpatokal
I've generally found it useful to not speak any Japanese in these situations, it makes interrogating you too much a hassle for them...
My experience has been that if you don't live in Japan but speak Japanese, don't admit that you do. That sets you up for far too many questions. On the other hand, if you do live in Japan (legally), and speak (at least some) Japanese, by all means, speak it. You'll be treated better and will be let go quickly.