The use of a comma in direct speech...

[Tom]

Hi

I just finished a novel by an American author. One of the things that caught my attention was the use of a comma at several places before the adverb in direct speech. Please see:

[SkiIucK]

I am no expert, but I believe such commas are placed to put special emphasis on characters' feelings.

[Kitosdad]

SkiIucK my friend, as you rightly say, the comma makes all the difference to the way the text is read.
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Keep it simple ... Keep it interesting.

[Kitosdad]

She said, sadly, "I really don't know what you are talking about!"

I also think that the exclamation mark should not have been used at this moment.
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Keep it simple ... Keep it interesting.

[Mister Micawber]

.
I'd say it is the author's choice. Sometimes he wishes a pause for consideration. It may also be period style. I have noticed that James Boswell, in his Life of Johnson, places a comma before all 'that'-clauses.
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Native English teacher at Mister Micawber's