Sometimes they're interchangeable, sometimes they're not.
Today I'll explain the difference between dashes, commas, and parentheses.
Stever Robbins, the host of the Get-It-Done Guy podcast, has been writing a book, and his birthday is today. He told me that his birthday wish would be for me to do a show to help him understand the difference between dashes, commas, and parentheses because it keeps coming up in his writing. Well, Stever, it's an odd thing to want for your birthday, but here it is.
In general, you can think of parentheses, commas, and dashes as a continuum of marks. Parentheses are the quiet whisper of an aside, commas are the conversational voice of a friend walking by your desk, and dashes are the yowl of a pirate dashing into a fray.
Let's start with those quiet parentheses. You use them to surround something that seems a bit out of place in the sentence—an aside, a clarification, or a commentary. Sometimes when you go back to edit your first draft, you'll find that you can rework your sentence to include the parenthetical statement or simply delete the things in parentheses, unless they're something like irreverent quips that are an intentional part of your tone.
Here's an example of one way to use parentheses to add additional information:
The 30th anniversary of the eruption of Mount St. Helens (May 18, 1980) brought back vivid memories of ash and darkness.
The date (May 18, 1980) is in parentheses in that sentence. It's something you want to tell the reader, but it isn't a necessary part of the sentence. If you leave it out, the reader still gets the whole point you wanted to make about revived memories because of the anniversary.
The date isn't enough of a dramatic statement to merit dashes, and if you want to leave it in, another good reason to use parentheses is that the date already contains a comma between the day and the year, so to surround it with commas would make the sentence difficult to read. No excitement. Already has an internal comma. That leaves parentheses as the obvious choice.
Here's one that's a little different:
I'm heading out (movie night!), but I'll call you in the morning.
"Movie night" is more of an aside or comment than a clarification. "Movie night" is so far removed from the flow of the sentence that you wouldn't want to use commas around it. You could use dashes. It doesn't seem like enough of an interruption or a dramatic statement to me to merit dashes, but it's a judgment call. You could write the sentence a different way, of course, "I'm heading out for movie night, but I'll call you in the morning," but it doesn't have the same friendly, happy feel. Parentheses seem right here.
The examples I've given both have sentence fragments enclosed in parentheses, but you can also enclose whole clauses.
Next: How to Use Dashes