How we change what others think, feel, believe and do
Simply watch what is going on. There are two ways to observe: watching and engaging.
The first method of simple watching is to stand well back and not become involved at all with the worker. This may even be via a video link (which may be recorded, of course). The second approach is to get closer, asking the worker about what they are doing and perhaps even having a go yourself. As an alternative to on-the-spot engagement, you can make notes and ask the person later about the detail. One way of bridging the time gap between action and questions is to video the proceedings and both look at it later.
A way of 'hiding in full view' is to do the job yourself. in this way, you both get a hands-on feel for the work and also may observe and question experts without them knowing. Of course, you must consider ethical concerns when doing this.
Both what and how may be the target of observation. You may also want to understand how people are thinking about what is happening, how they are constructing meaning, etc., in which case some associated questioning is necessary.
Observation is better than many other methods in that you actually see what is going on, and can thus pick up on aspects of the jobs that the person involved may not mention in an interview or self-report.
The principle of detached observation is that any interruption corrupts and contaminates the work. Even the fact that the other person knows that you are watching them may well change what they do or how they do it.
Engaging with the worker, however, allows much greater detail to be captured. Asking 'why' can uncover aspects of the job that are not covered elsewhere. Although doing this on the spot does interrupt them, it enables more immediate explanation and may well lead to more detail being discovered than asking later.
Some jobs suit either detached or engaged observation. Where the worker is concentrating, interrupting them may well spoil the whole flow of the work. It is, for example, difficult to ask many questions of a police officer as they are making an arrest. Where you do ask questions later, the sooner you do it, is usually the better.
Many other jobs take so long and act across so many contexts that direct observation of the whole job would be very time consuming and difficult. In these cases, observation of critical elements may be more useful.