What characteristics should an interpreter own?
I think he should be an expert of everything.
Why is that?
Because English is nothing but a tool which is used when people may negotiate in international conferences, probe into physics, release some information in a press conferrence, and do anything we can do with language. Therefore, an interpreter has to know some expertise in the field he / she serves as an interpreter. Otherwise he / she can not carry out his / her work smoothly.