Find out how people act in real life by using a non-obtrusive observation technique.
Try to watch people without talking with them. While observing, make notes of peoples activities, interactions and environments.
A spot where you can see the users without being noticed.
A note taking form that will give structure to your observations and allow you to code and count behaviour easily.
Sketches of the environment in which the behaviour takes place.
A willingness to stick to the facts and delay interpretation.
Fly on the wall observation is a very useful method for situations in which people act different than they say they do, or when the presence of a researcher might cause them to act in awkward ways. A special form of fly on the wall is 'day in the life' in which users are shadowed during a full, regular day.
Universal methods of design (Martin & Hannington , 2012) spend a couple of pages about observation (in general) and fly on the wall specifically. In the Dutch version: chapter 57, p172-173 and chapter 42, p90,91
Observing the User Experience (Goodman, Kuniavsky, Moed, 2012) also has a chapter on observation and field visits. Chapter 9, p211-241