Internally, the character of Tom Walker is developed through the narrator's description of his greed and how he is such a miserly character. Note how this is achieved in the second paragraph of the story:
He had a wife as miserly as himself; they were so miserly that they even conspired to cheat each other. Whatever the woman could lay hands on she hid away: a hen could not cackle but she was on the alert to secure the new-laid egg. Her husband was continually prying about to detect her secret hoards, and many and fierce were the conflicts that took place about what ought to have been common property.
There is clearly exaggeration used here by Irving in the way that he presents Tom and his wife, as suggesting that they were so miserly and so much in competition that they even tried to hide eggs from each other is rather an extreme action. This however shows the internal development of Tom's character through presenting his greed and miserly nature, for if he is already so greedy, he will be prepared to do anything to gain wealth. His external development is shown through his conversation with the devil and how he is willing to risk and trade anything, even his soul, in order to have wealth and to use it in a way that will cheat others and add to his own riches.