They write up their field notes religiously, out of habit. There will be a great temptation to avoid keeping a system atic record of what you observe. But without extensive notes, your memory will fail you and you will often report what you imagined and perhaps wished had happened. To be scientific you must report exactly and only what you observed. Interpretation proceeds as you write up your observations.
Interpretation can work in two ways.
If you have a theory that identifies cause and effect, you can determine whether the theory applies to what you have observed. One student did a study of musicians playing on the street or in the subway. His observations were examined to see how well they fit with the theory. For example, many of the musicians exhibited a business orientation, displaying signs with their name on them and selling tapes of their recorded performances. They intended to show listeners that they were not hacks, collecting a few coins, but legitimate, skilled musicians.