Interpreting requires far more than transposing words from one language into another. Interpreting is a complex process, which involves the interpreter extracting the meaning behind the words in the source language (decoding), transferring that meaning from one linguistic context to another, and expressing it in the words of the target language (encoding). At the same time, the interpreter has to be on the lookout for any meaning expressed other than in words, for example, by tone of voice, intonation, facial gestures, body language, etc (paralinguistic features). The aim is not to lose any of the meaning whilst expressing the message in idiomatic (natural-sounding) language. And all of this has to happen instantaneously! This goes some way to explaining why interpreting is such a mentally tiring process.
In the legal context, accurate interpreting is crucial. In addition to a very high level of linguistic proficiency, a legal interpreter requires a thorough grounding in legal terminology (in both languages), a knowledge of the legal systems of both countries (and where legal concepts differ from one legal system to another, such as, for example, negligence, recklessness or manslaughter) and familiarity with legal procedures.
A good interpreter will be alert to the possibility of misunderstanding (for example, where the meaning of a word is ambiguous) and will alter the parties to this.
A good interpreter will realise when cultural factors come into play which may prevent understanding, and, again, will alert the parties to this.
A good interpreter will know the boundaries of their role, and will not exceed them.