Interrupting is difficult. I imagine it’s always difficult, even when everyone is gathered together in a room or on a stage, because when you’re putting on a show you all have your assigned lines and it’s natural to take turns saying them. It’s hard to leave the “You go, now I go” dance and move into listening for the sentence to come close to the interrupt point and start speaking while the other person is still talking. It’s not made any easier when the lines are delivered remotely and there’s no one else around to actually interrupt. In editing I’ve erred on the side of making sure every word is intelligible (as it is when you read them) and never had any voices overlap. I sometimes wonder if that was the right decision.
Interrupting is also a marker of status, and a sign of politeness. McGonagall is very proper and rarely interrupts anyone. Severus is abrasive and interrupts all the time. Most other characters follow social norms and freely interrupt those below them, occasionally interrupt their peers, and almost never interrupt a superior. It’s interesting to note that Harry Potter will freely interrupt anyone he’s talking to when he feels he has something to add. I asked a friend who didn’t like the fanfic what turned her off, and she said it was the way Harry showed disrespect to McGonagall. I was a bit surprised, because I didn’t feel he had (at least not egregiously). But I had been reading under the assumption that Harry was a person, and so him treating McGonagall as a peer didn’t shock me. My conversational partner was a parent, and viewed Harry as a child, and for him to interrupt her or question her was too great a violation.