Once you have decided the types and number of people that are in your story, you need to define them. The way they look, how they may act in general, and even how their society might function. I may have already gone over some of this, but it never hurts to go over it again. ;)
I'm a bit of a traditionalist when it comes to the people that occur in my stories. Humans are typically humans. They live in some sort of monarchy, since that is the time frame I tend to write in, that may or may not be central to the story. If it is, it's because the king is a primary character of the story, if it's not his story. If it's not, I don't usually worry about it until it needs to be addressed in the story.
Where I like to play is with the other races of people that I use. In one story the elves look just like humans, except they have feline or cat eyes. To get by in a human society they have to hide their eyes. In another story the elves are normal elves; slight builds, long, light, fine hair, and pointed ears. They typically live in some sort of monarchy, just like my humans, but that's normal for elves. Sometimes they have a limited monarchy or republic, but Monarchies are the norm.
My dwarves are often shorter than humans, but they could be stocky or slightly built, depending on their trade. Obviously, the miners would be the short and stocky ones, but if they don't need the bulk for their work, they sometimes look like short elves. Because they tend to be workers, I like to have my dwarves in a communal or tribal society. Everybody works, so everybody earns the same regardless of status, or they live in small groups that work one area or around one mine that is their "tribe's" territory.
When I have animals, or animal-human hybrids, they are completely different for each story.
All that being said, these differences make the interactions of your people. Whether both(all) races live in a monarchy, then perhaps they have the same problems and can commiserate on it. If they don't, then one side could spend their time trying to take over other races so that all would be under one rule. Whether by arguing how much better their government is, or by going to war with the neighboring lands.
The difference in height between a human and a dwarf would make humans more likely to look down on dwarves. Whether that translates to them being considered "inferior," or just the height difference, comes out in the story. For the elves that look just like humans except for their eyes, anyone who might hide their eyes would be considered an elf, and subject to the prejudices or benefits of that story line.
To sum up, the more details you have about your people, the more you know about what will happen when they come into contact with each other, and how that can drive your story and define your characters.