Are you a best man? If so, I would ask the couple what they would like you to do. If they are going all-American style, then the traditional western best man duties can apply. If they are going with a mixed style, clear everything with them first. If they are going all Japanese, also ask, as what is a "Japanese" wedding (unless it is very traditional) is very open to interpretation by the couple involved.
If you are going to ask someone to be a best man, don't be shy to tell him what you want. Good luck