In "An Astrologer's Day," what details and techniques does Narayan use to describe the astrologer?

The astrologer is an impressive character. As the narrator says, if he had stayed in his village

...he would have carried on the work of his forefathers--namely, tilling the land, living, marrying, and ripening in his cornfield and ancestral home.

But when he was forced to flee the village, he was able to develop a whole new personality and survive in a densely populated urban environment by using his intelligence. The story suggests that many people who live in primitive rural conditions could likewise develop all sorts of hidden talents if they had the opportunities. 
The astrologer lives by his wits. He knows how to put on a show to attract passers-by, and he knows what to tell them, even though he is well aware that he has no mystical knowledge. He has no education and is probably illiterate. But he has "street smarts," "moxie."

He had a working analysis of mankind's troubles: marriage, money, and the tangles of human ties. Long practice had sharpened his perception.

He only collects small coins for his consultations, no doubt because most of the people who stroll in the park for recreation have little extra spending money. It is apparent that he must sit for long hours in order to collect enough to keep himself and his family alive from day to day.

...his eyes sparkled with a sharp abnormal gleam which was really an outcome of a continual searching look for customers, but which his simple clients took to be a prophetic light and felt comforted.

Money is of great importance to him in his precarious profession. In addition to his superior intelligence, he is courageous and determined. When he is dealing with Guru Nayak and his life is in imminent danger, the astrologer still insists on haggling over money. He brings every single coin home to his wife so that she can buy food for the family. Evidently he is a devoted husband and father.
"An Astrologer's Day" is a study in the vicissitudes of life and of one type of adaptation. The astrologer would have been an ignorant peasant if he had remained in his village, but the big city forced him to adapt to entirely new conditions, and he managed to find a niche in which to survive, marry, and reproduce. He is a survivor. We can identify with him because we all have to learn to survive in this world by adapting to our environment.