What you should care is a behavior. You should be open about what is expected. People change behavior all the time depending on the situation/group/company/context.
People can change the behaviour for brief periods. But who they are day in day out is going to be pretty consistent. Telling people how I would want a good candidate to behave during an interview doesn’t help at all with candidate selection. An inclination towards saying things they think people want to hear is a characteristic I’d like to select out of my candidates as well, so perhaps I’ve been killing two birds with one stone here…
> People can change the behaviour for brief periods.
This is a false assumption. Especially generalizing the behavior in such an adversarial setup as a job interview to a regular day to day work/life.
> Telling people how I would want a good candidate to behave during an interview
You should tell them the rules of the game. The thing is, with interviews, there are already predefined assumptions, such as not knowing something takes a point from you, so people avoid this. In your case, you are altering these assumptions without disclosing it. So people might already had changed their behavior for the interview specifically - avoiding admitting not knowing something.
