Lets suppose an American guy goes a friend's birthday party. Both of them are male, and both of them are 28 years old. So he knocks the door, the other one opens and: What happens next?

And what would happen if they were both girls of that age or one of them boy and the other one a girl?

(I know that some people may find this question weird if they suppose that I'm not from another country. Since I am from another country, I will appreciate informative answers. Thanks.)
One more thing: do you ever kiss each other on the cheek?