I don't like it when TV shows put dialogue in the middle of a song. That seems like a bad idea in general. "Hey, we're doing a musical number! You know what it needs? Dialogue that isn't set to music!"
Seriously, if you need your characters to chat for a bit, have them do it after the song, not in the middle of it.
The worst case scenario is when the talking voice and the singing voice are done by two completely different people. This becomes extremely obvious when the character interrupts their own song and abruptly changes voices. My reaction is usually, "When did your voice become deep and silky?" or "Hey, what happened to your accent?".