Atticus is like a mirror, that reflects reflection, truth and flaws. How is this metaphor true to particular characters ( eso adults) in the text?
This statement is proven in most of Atticus' private "mini-lessons" with Scout - but I think the easiest place to look first, is the courtroom scene.
When Atticus questions both Bob and Mayella Ewell, he does so in a way that shows both to be lying, first, and truly reflects who they really are. When he questions Tom Robinson, he is able to present him as human, to a courtroom full of people who have never thought of him as such.
I encourage you to re-read chapters 17-19. Watch the way Atticus does not change his demeanor from anything we've ever seen of him previously in the book. Watch his patient and respectful style - as it picks apart Bob Ewell piece by piece, so slowly and methodically that Ewell doesn't even see it coming. Watch as he rattles Mayella into crying - knowing that he's forcing her to be truthful and knowing further that she cannot do anything about it. Then watch as he talks to Tom Robinson - and for the first time in the story - we (the readers) have an opportunity to love and feel sorry for him.
Atticus, by his very personality - which is authentic and full of integrity always, brings out the truth in people because he commands it by his own lifestyle. He puts people at ease because he isn't tricking nor conniving. He is simply having conversations... seemingly casual ones.