If you use ellipsis points at the end of some dialogue to indicate faltering or trailing-off speech, you do not then have to announce in the narration that the utterance has "trailed off." It's evident from the punctuation.
If you use an em-dash at the end of some dialogue to indicate sharply cut-off speech, and the next line is another line of dialogue, you do not have to announce at any point that the second speaker has cut the first speaker off. The two lines of dialogue and their punctuation work aurally, just as in a playscript, and the reader doesn't need any help from the narrator to get what's happening.
( For example... )
A lot of the time, "said" tags are unnecessary; we know who's speaking from context, from interpolated description ["Okay." His pained expression belied the assent. "Yes"], and from the content, syntax, and vocabulary of the dialogue itself. But "said" tags do serve a rhythmic purpose and help keep the reader oriented, and as long as they aren't awkward or excessive they're close to invisible, hardly registering on the reader's conscious mind, as integrated as punctuation. "He trailed off" and "he said, trailing off" and "She cut him off" and "she said, cutting him off" make the "said"s visible to no good purpose. When they appear frequently throughout a piece of fiction, they amount to a rhythmic crutch; to me they loom larger each time, and become annoying narrative tics. It's like the TV commercial where the guy's in the job interview talking about his qualifications and what the interviewer sees and hears is this droning guy in the background and the stain on his shirt SHOUTING IN CREEPY GIBBERISH!!!! He can't focus on what the guy's saying, because the stain distracts him so much.