First, commas indicate pauses, so put them where a speaker or reader would naturally pause.
"Look, Jones,"
That one is important, because there's always a bit of a pause between a command and a name.
Second, imagine how your speakers are moving physically. Does Dan just point briefly? Does he only mean the castle will belong to Jones? Does he make a sweeping gesture to take in the castle, the grounds, the cliff, and the sea? When does he point?
Gestures mean pauses, for dramatic effect. A short pause is a comma, while a longer pause can be indicated by a period or an m-dash. If you want to emphasize the castle (as opposed to the cliff, the sea, the horses, or the knights behind them), use italics. (I'm changing "Dan" to "he" only so there's no confusion about starting a new sentence.)
"Look, Jones." He pointed at the
castle. "This will be yours someday."
"Look, Jones. This — " He
pointed at the castle. " — will
be yours someday."
"Look, Jones." He pointed at the
castle. "This will be yours someday."
I sometimes trip over the m-dashes, but generally speaking, you end your quoted material with the m-dash, put your interrupter narration in the middle as a complete sentence, and then pick up the quoted material with an m-dash and a lowercase letter, not a new sentence.
Third, to answer your question more specifically, to use commas around interrupter narration:
- The narration itself, without the quoted material, should not be a
complete sentence.
- The quoted material should continue
a sentence.
"Look, Jones," said Dan, pointing at
the castle, "and you'll see what I was
speaking of before. This will be yours
someday."
If the quoted material starts a new sentence, then end your sentence at the end of the interrupter narration.
"Look, Jones," said Dan, pointing at
the castle. "This will be yours
someday."