In V.S. Pritchett's "The Fly in the Ointment," the father and his son, Harold, are very different characters. Harold acts as a foil to his father in the story—by comparing Harold to his dad, we get clearer picture of the older man.
A foil is:
...a character who contrasts with another character...in order to highlight various features of that other character's personality...
The reader first meets Harold who tries not to upset his father, as seen when he arrives at the factory which is going out of business:
Better not arrive in a taxi, he was thinking. The old man will wonder where I got the money from.
This comment lets us know that money is an issue in this family—an example of foreshadowing; it provides hints about important events to follow.
Harold has come to offer his father moral support during this tough time. We can infer the kind of a man Harold is—best seen in how his father acts, and how Harold still offers his help:
"Come in, Professor," said the father. This was an old family joke. He despised his son, who was, in fact, not a professor but a poorly paid lecturer at a provincial university.
Harold is aware of how his dad feels. He makes polite small talk and tries not to annoy his parent. We see Harold's true character in his willingness to put aside old pain and arguments to be there for his dad:
Suddenly all the money quarrels of the family, which nagged in the young man's mind, had been dissolved...He was overcome by the sadness of his father's situation...I must see him. I must help him.
Harold still loves his father, and he suffers at the thought of his parent going through such a hurtful ordeal.
At the other end of the spectrum, now we learn a great deal about Harold's father. It's worth noting that sometimes when someone is feeling powerful and successful, he/she may not be very nice. However, a change in character might emerge when things start to go bad. Perhaps this is why the father at first seems so nice to this son he hates:
Hello, old chap. This is very nice of you, Harold.
This appears to promise a warm and gentle exchange between parent and child. However, it is not long before the father cannot help himself, and he criticizes Harold's baldness:
Do you know, you're actually more bald at the back than I thought. There's a patch there as big as my hand. I saw it just then. It gave me quite a shock. You really must do something about it. How are your teeth? That may have something to do with it. Hasn't Alice told you how bald you are?
The father is cruel. Why should he find fault with Harold's physical appearance? And there is no joking here, no mutual respect. We begin to see the father's character much more clearly.
At the end of his financial rope, the father swears off money...he has had it, he tells Harold.
"Money's been my trouble," said the old man. "I thought I needed money. That's one thing it's taught me. I've done with money. Absolutely done and finished with it. I never want to see another penny as long as I live."
It seems that the man who was so critical of a son who makes such little money may have changed—until his son says he wishes he could raise money for his dad—who is voracious in his response:
"Raise it?" said the old man sharply. "Why didn't you tell me before you could raise money? How can you raise it? Where? By when?"
Harold cares about his father; his father cares about nothing but money and himself. The two men are very different!