If one wanted to say something like
no commas: Hello Pete, how are you?
or
commas: Thanks, Julia, this will be helpful!
would he need commas separating the different elements in the interjection or just the comma separating the interjection from the rest of the sentence/clause?
You would need commas before the interjection (if it is at the end of a sentence); before and after the interjection (if it is in the middle), including names; after, if it is at the beginning of the sentence.
Examples:
Celia, would you take that call? (Interjection at the beginning) Will you, Aisha, do that assignment for me? (Interjection in the middle) Yes, Doctor, I will. (Interjection in the middle) I can, if you like, complete the project for you. (Interjection in the middle) I can complete the project for you, if you like. (Interjection at the end) I can do that for you, Alicia. (Interjection in the middle)
You can find more good rules here: http://www.grammarbook.com/punctuation/commas.asp
