a plumber doesnt ask his customer how to plumb. just which pipes need fixin! doesn't go fix the toilet if the shower is broken :S
In that case the plumber could and should ask where the sink is going, even if he was only told to move the toilet.
Why? Because maybe the sewage line is over there and about to be covered over with concrete and they’ll have to tear up all of that to move it in two weeks, but the sink hadn’t arrived yet so no one told him that would move either.
The point is not that the plumber is asking his customer how he should do his job, but that he can be more valuable if told the whole scope and impact of his job.
Similarly you can’t just ask a user “how could this screen work better”, you need to watch them work so that you see that that screen isn’t even the problem - they’re clicking off of it 15 times every call - but you didn’t even know to ask about the other page.
Why would you ask that anyway? You've already assumed the problem and solution at that point.
You ask how it's going, very generally and let the conversation lead you to their pain points.
You don't ask about the UI, which they are not an authority on. You ask about their subjective experience, which they are.
A few months ago I heard a talk by the chief product manager at a photo album printing company where he talked about an in-depth series of customer interviews they'd recently conducted.
His advice was to keep asking why until you get to the root of why someone did something. However, if the answer you reach 'Because I don't want to die alone', then you've gone too far...
Also, 5 is just the "suggested" number. More or less may be needed. If you reach 'Because I don't want to die alone' in 3 whys, probably no need to try for 5...