DOCUMENTATION
As previously mentioned, you cannot over do the documentation. You should write down what the users told you during your interviews. In addition to the advantages already mentioned for using a notebook, writing notes in your notebook is a lot less distracting and intimidating for the users than banging away at your laptop keyboard or even worse filming the interviews.
You should also write down everything you know about the subject system. If it seems appropriate you might consider taking a picture of the computer and screen. If you suspect that physical security has been breached, it is an especially good idea. You are now ready