I've often heard comments like "this person may not be a good fit for this company / role", and have felt that way about some candidates I have interviewed. 

This is true of candidates evaluating companies as well, when they feel that a particular company or a role may not be a good fit for them.

Either when we're evaluating a candidate, or a role for ourselves, or a change in situation in general for us, like moving to a new city or a new home, the questions to ask are:

Will my current way of doing things help me succeed in the new situation? If yes, it's a good fit already.

Am I willing to change the way I do things to be in line with what it needs to be to succeed in the new situation? If yes, this is not an immediate good fit, but there's potential.

Will what is needed to succeed in the new situation change to match the way I currently do things? This almost never happens, but if there is objective evidence of this, this has potential as well.

But we are often in the last category where we are unwilling or reluctant to change how we do things and that's not what's needed to succeed in the new situation. Recognising this is recognising a lack of fit.