I'm currently reading a book published in the seventies (1975 to be exact) and ran across the following snippet of dialogue. For context, this is a scene where a medieval lord is speaking with the heir of a rival lord. "There is no point in continuing this war, really. Your defeat must have convinced you of what a mistake your invasion was. (Let me finish!) Breimen lost the best part of its army." Note the parentheses. I'm confident they aren't necessary, and I don't particularly care for them as a formatting choice. But what about you? Would today's editors discourage such usage?
Yeah, that's stupid. I would use em-dashes to bracket a paranthetical "thought" within the dialogue, but the example appears to an exclamation in reaction to being interrupted, which would seem to call for a beat or for the other character to break in and do something.
The only way I can see it working is in a comedy making fun of overly ornate antique methods of writing and speaking.
I agree with Homer Potvin. It's intended to convey a break in the flow for a somewhat forceful (I presume) interjection. I think I would have done it with small caps.