Generally, no, the letter after a comma in dialogue is not capitalized.
* Example: "Hello," she said, "how are you?"
However, there are a few exceptions:
* When the dialogue starts a new sentence:
* Example: "Let's go," he said. "I'm hungry."
* When the dialogue is a proper noun:
* Example: "This is my friend, John," she said.
In short: The capitalization rule depends on whether the word after the comma begins a new sentence or is a proper noun.