If your text is to flow naturally, the typographical conventions need to be observed as well. It’s not just about getting the wording right.
Starting a new speaker on a new line is definitely the norm. Even on websites and in brochures and other places where space is tight.
- when the speaker changes: “Hi!” said John. “How’s life?” Pete replied. “Great.”
=> I’m losing track of who’s said what.
- when someone starts speaking (even if you introduce it with “Gerben adds” or something like that)
I’ve yet to see it done in a Dutch novel, though. But it’s not simply a question of space not being at a premium in a book. Quotes and interviews in quite lengthy articles where space doesn’t matter are prone to run lines together too.
Prevalence: high. Strangely, given that it’s not really good form in Dutch either.
Frequency: high. Generally turns up consistently throughout a document.
Native: no. Although if you’re using CAT (software to assist human translation), you often can’t control and change the line ends and other layout.