Interpreting does not merely consist of having a thorough knowledge of two or more languages. An interpreter conveys a message through processing information from one language accurately into another language, maintaining the original register (formal, colloquial, slang), tone and style, and being as faithful as possible to the intentions of the speaker.

Therefore, a person would likely not be a good interpreter if he/she does not:

work well under stress

have an in depth knowledge of the vocabulary, including slang and jargon, of the source and target languages

have a broad knowledge of the grammar of both languages