Atticus treats Walter much like he treats his father and his children: with great respect. He engages Walter into a conversation that Walter will feel confortable talking about: farming. When you read the conversation the two have, it sounds like two adults talking. Atticus does not judge his guest and will bend over backwards to accomodate Walter. When Scout rudely comments on Walter's eating habits, it is Atticus that stands up for Walter at the dinner table by letting Scout know what she said was not proper. He does not say a thing (he does so that he doesn't further embarass Walter) but Scout knows that she is in trouble if you take a look at her response afterwards.
His treatment of Walter as an equal is similar to the way Atticus prefers to treat everybody. We can find a direct correlation between Atticus having a real conversation with Walter and Atticus defending Tom Robinson. Atticus isn't as concerned with public perception as everybody else is - a lesson he's hoping he's passed down to Jem and Scout when he instructs them to walk in other people's skin.
Atticus treats Walter with respect when he comes to visit the Finch home. Walter's family is poor and Walter has had to stay out of school for long periods to help his family survive. Atticus listens carefully, and the man and boy "talk like two men."
Atticus, along with Calpurnia, continue to show their guest respect. In Scout's opinion, Walter breaks the standard rules of decorum when he asks for molasses (a treat) and then pours more than is polite all over his food: "He would probably have poured it into his milk glass had I not asked him what the sam hill he was doing," Scout exclaims.
Atticus and Calpurnia will not allow such a dressing down to company. Scout is quickly chastised for her lack of manners. Atticus extends the hand of humanity and human dignity to everyone.