It's true, people change. It's also true that people don't change. You're always who you are, but who are you?
If activities and actions can have an affect on you and cause you to react, do they change who you are? Are you truly like Schrodinger's cat, never known until an action is taken?
On one hand, we are but a mix of chemicals that react to produce actions. Like baking a soufflé, any small deviation from the recipe matters when you’re reacting to a negative comment. Have you had your morning caffeine? Are you niccing for a cigarette? Maybe you’re in a food coma?
On the other hand, we have our born-and-raised behaviors and perspectives that define what we consider acceptable actions. Generally, these tend to be the maximum extents to which we might react in a certain way. How you react to receiving a gift will be similar whether you're in a plane or on a train.
While both of these occur and exist with relative consistency in a person, the amount of energy and self-awareness it takes to pull these from the sub-conscious to the conscious makes it a rare occurrence for someone to be able to accurately predict how they will react in a particular situation. It's easier to just ride it out and see what happens. That's what our brains are trained to have us do anyway, only pulling in conscious thought when something really gets weird. That said, even if we could know what we might do, we still won't know how that chemical soufflé will turn out in a given moment.
That shouldn't stop us from trying though. The best tool we have is reflection. What did we do when? Why do we think we did that? How was it different than last time? Just like Zeno's tortoise we may never quite make it to always being right, but we can certainly get close. This is an innately human quest. It’s why yogis meditate and some Native Americans go on vision quests.
As a designer, awareness of the influence of reflection can help you better empathize with your interview subjects. Understanding, implicitly, the challenge of forecasting one’s own actions should make it readily apparent as to why you should simply observe an interviewee’s actions in a testing environment, rather than asking them what they think or why they reacted a certain way. By even posing these questions the situation changes, creating yet another layer to what might have seemed like a straightforward “recipe.” In future moments, now the interviewee will have potentially already put thought into what might have otherwise been a relatively instinctive and telling action or reaction. Now they know what they thought they would do. The interviewee may even feel some obligation to you, the interviewer, to uphold their previously-made, contextless decision, though now that they are re-thinking it, in context, they desire to take a different approach. How they handle that is anyone’s guess.