On the surface, this extract doesn't tell us much about Mr Sir, but you've probably begun to form an opinion about him. If we read between the lines, we can tell that he likes to make an impression and wants to be seen to be in charge. He is sitting "with his feet up on a desk" and has made his appearance even more striking by wearing sunglasses and a cowboy hat indoors.
Sometimes people who act like this can like power a bit too much, and can be bullies. Could Mr Sir be a bully?
When we consider the can of cream soda, we can say that the fact that Mr Sir fails to offer either Stanley or the bus driver anything to drink after their nine-hour, cross-desert journey suggests that he is a selfish man. That he keeps his own drink in full view of Stanley and the bus driver further suggests that he is cruel and likes to see other people's discomfort.
We learn that Mr Sir stopped smoking recently. We could then expect him to be bad- and short-tempered. The tattoo of a rattlesnake, a dangerous reptile that is supposed to have a bad temper, adds to this impression. The sunflower seeds are, we are led to believe, there to help him as an alternative to cigarettes, but we don't get the feeling of optimism that they'll be successful.
So there we have it; Mr Sir is a bad-tempered, selfish bully!