Why does the director spend time talking to the actors individually at callbacks?

The director wants a chance to get to know the person. When a director is casting, they are keeping a lot of things in mind such as whether this is a person who takes direction or whether this person is and enjoyable person to work with. Hope this helps :)

To get to know them better. If I were a director I would be looking for someone who is a nice person off-screen because you don't want to deal with a rude person, or someone who doesn't like criticism for example...

What it's saying is that colored pieces of canvases is a piece of art