Interrupted Dialogue

No comma, you just need an em dash: "I didn't think that you would—"

This is both the way I do it, and seemingly every book I've read does it:
"Yeah, but..."

I do something else that I don't know if anyone else does. It's my system, and I like it
If I am interrupting a sentence, and the next utterance is a new or altered thought, I use the "..." both before and after:
"Yeah, but...," he started, then said, "...ah, nevermind."
But if I am only pausing, or stuttering a sentence, I use the "--" in between:
"Yeah, but I -- uh, forgot what I was going to say."

I agree with Maryn. I use ellipsis (...) to indicate trailing off and em dash ( — ) to show an interruption.

It's really hard (at least for me) to suss out all the various citations in CMoS, but it seems to allow both ellipses and dashes:

"What the heck did she mean . . . " Kara paused to catch her breath. ". . . by all that nonsense?"

"What the heck did she mean--" Kara paused to catch her breath. "--by all that nonsense?"

Well, I guess I'm backwards to everyone's theory here. But I'm stubborn, and I like my method.
And I'm pretty sure many of the books I've read have "..." as an interruption indicator. I'm creative, but not creative enough to completely invent some new system, I don't think.

Maybe our posts were too close, but as I indicated, I don't think your method is backward or against CMoS.

Without the comma, ellipses are in common use for interruptions by many writers. Stephen King employs both dashes and ellipses to interrupt in the same novel--sometimes in the same chapter.

Chase -- good, thanks. I am far too lazy and stubborn to go back and change my system, unless of course some big-boy publisher gives me an advance and insists on altering it. "Right away!" shouts my sell-out soul.

Chase said:

Stephen King employs both dashes and ellipses to interrupt in the same novel--sometimes in the same chapter.

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"Just because you see someone else do it, doesn't make it right. I mean, would you follow this Stephen King fellow off a cliff if he jumped first?" (channeling someone's mother...)

BethS said:

"Just because you see someone else do it, doesn't make it right. I mean, would you follow this Stephen King fellow off a cliff if he jumped first?" (channeling someone's mother...)

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Good advice, Mom, 'cause most of us lemmings headed for the cliffs do use the more popular dash method, but I've seen other writers also use ellipses. They're growing in popularity for dialog, leaving dashes for narrative.

CMoS isn't the be-all and end-all of writing styles, but it does seem to give some credence to alternatives for dashes when interrupting dialog.

"We live east of Hoback. Shaye…” He gestured at his wife. “…is a trouper. We’ve lived in a trailer five years." From Don't Run by JJ Pitts.

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Maryn said:

I concur, no comma, just the em dash. Use another if they proceed mid-sentence.

"I didn't think that you would--"

"Shows what you know!" (or a passing siren blocks their conversation, or whatever interruption you have)

"--ever sink this low," he said.

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I do it this way. It's a great thing that the forum taught me. It makes sense too, because the cutoff is at the beginning of the following statement, if you think it through.

Daniel, who thinks Maryn is cool.