I have tried interpretation myself, be it simultaneous or consecutive. It feels as if you are a split person, having the left side working quite differently from the right side of your brains! The ears would listen in 1 language, and the mouth would reproduce the words in another language, almost at the same time.
A good interpretor does not allow for a lag in the speech. He or she will start almost at the same time as the speaker. It is as if the interpretor is the speaker! It takes a lot of knowledge on the topic as well as understandingof the speaker. If the interpretor has worked with the same speaker for some time, he or she will be able to guess what the speaker want to say in the next sentence. This takes a lot of practise, and not to say, much more energy and concentration on the matter.
Why more energy? A good interpretor not only translates verbally the words, he or she would transfer the emotions of the speaker to the audience too!
Not all translators can be interpretors. For translation, you have all the time in the world to slowly “digest” the source and “spit out” the translated words. It is not so for interpretation. Even for consecutive interpretation. You would only have that sentence length of time to process the source and form the correct words.
It is a very challenging job and I did enjoy my experiences as an interpretor. So, for those who would like to try out interpretation, you can start with listening to some audio and translate on the spot. You can record what you have interpreted, so that you can check back on your translation later on.