This question concerns Chekhov's uses of characterisation, or the way that he builds up a convincing portrayal of the characters he creates. Of course, characters are shown not just through what they say, but also what they do and how they appear. Note for example how in the opening lines, the banker is shown to be walking up and down, thinking of the past. His actions clearly reveal that what he remembers concerns him. Consider another example of the banker in his youthful days and how his character is revealed through his actions:
The banker, who was younger and more nervous in those days, was suddenly carried away by excitement; he struck the table with his fist and shouted at the young man...
The narrator tells the reader that the banker was "younger and more nervous in those days," but his excitement is conveyed through his action of striking the table with his fist and shouting and the young man opposite him. Later on the narrator refers to the banker as "spoilt and frivolous, with millions beyond reckoning." Such details are clearly matched by the impulsive way he bets so much money. This is of course something he comes to rue in the present, as he has lost much of his money, and his confidence, and to pay his bet would be to ruin him completely.