When would you use parentheses vs. brackets?
Parentheses are a removable enhancement by the speaker or author; brackets indicate words inserted by someone who isn't the speaker or author.
Anything inside quotation marks is regarded to be a character's own words, even if it's in parentheses, unless it's in brackets.
Parentheses are a way of saying "This is removable and the sentence will still make sense, but it's an enhancement to the statement, made by the speaker or the author." In comic strips, before the advent of thought balloons, parentheses implied the character was either thinking these words silently or under his breath, for his and the readers' entertainment only.
Brackets are different. If a character named Maguffin says "I'll succeed or I'll die trying!", a bracketed quote in an essay about the story might read "[Maguffin will] succeed or [he will] die trying!" The speaker's actual words have been changed for the sake of clarity. In text, brackets are usually reserved for an editor's note, or some other interruption by someone other than the author.
Brackets are seldom used in literary writing, and are more useful in math or science notation.