Also (though not quite as strictly), if one character is doing something, and you switch to describe a different character doing something, that should be in a new paragraph. A whole bunch of back-and-forth actions between characters is just one of numerous exceptions to this rule. But it's good to keep it in mind.
"Wow, hey, ya know what?" Isaac asked.
Bob looked up from his workdesk in annoyance. "Whaddya want, Issac?"
Isaac winced, feeling sheepish. "Well, as you know Bob, I was working on this slide-rule problem and..." He then shuffled his feet, unsure how to proceed. "..well, I'm having a little difficulty."
"Whyzzat?"
"Well," Isaac whined, "for one thing, it's made out of Kevlar. Which is rather interesting, considering DuPont Corp hasn't even invented Kevlar yet."
Bob now glared at his new friend. Isaac was always going on about stuff like this. "Listen, Is', this reminds me of that story you were thinking about writing, about how history can be determined mathematically? And how this one math guy uses it to figure out that the galactic empire is heading for collapse? You remember that?"
"Hey now. That's no more silly than your short story idea about a blind spaceman songwriter. No one's gonna read that kind of silly libertarian fluff. People want serious concepts, future politics. Not crap from the pulp era. You're writing pure juvenile pulp, is what you are, with this idea." Isaac then folded his arms in an expression of superiority.
Suddenly the supervisor's basso profundo voice boomed out from the head office, startling them both: "Asimov, Heinlein! We're not paying youse guys to stand around and yakkity-yak! Back to your desks, the both o' ya!"
[This message has been edited by Nietge (edited June 19, 2006).]
There are some diferences between UK and US conventions on dialogue tagging, I think, but they're pretty minor.
In general, the simplest rule is that you should never have two different people speaking without a paragraph break in between. Non-dialogue lines should generally live in the same paragraph as the dialogue speaker they relate to.
Oh, yes. I agree with the rest about giving each speaker/actor his own paragraph. Please.
She worked for Dupont.
Saying a cardinal paid for the the materials and time Michelangelo used to paint the Sistine Chapel does not mean the cardinal painted it.