Interpreting differs from translating mainly due to the fact that there are no modification or rework possibilities.
You can nevertheless prepare yourself for the task. You need to study the topic carefully as there is always some level of information available, even in police interrogations.
It is advisable to familiarize yourself with the way the person that requires interpretation speakes – at least if there is only one who needs interpretation. Some tend to speak very fast while some speak unclear or too quiet. The audience typically listen to the interpretor, which is why it is important to agree on the way the interpretor should speak.
Mastering the richness of a language is a mandatory, but it is especially when translating technical words, or words of machine parts, when there is simply no equivalent in the other language. The interpretor can always ask for clarification or ask to explain or even draw a picture of the subject matter. What the interpretor always must do, is speak to its audience, clear and loud.
What the interpretor must never do is bring up his or her own opinion. Sometimes this can be really challenging, especially in court room interpretations, where your own morale might influence the work.