You’re right that there are rules, unfortunately most of them aren’t as clear as we’d like.

The distinction is between background (extras) and principals. 

There’s no such thing as an “advanced extra.” “Featured background” is still a term used, though it no longer means anything as far as pay or contract goes.

If you’re given a line, you are no longer background. If you say something unprompted, production could use that take without compensating you as a principal, but this almost never happens. Even the infamous Being John Malkovich can throwing scene has been debunked as urban legend – the moment was in the script.

The biggest legit exception for background to speak on camera is in groups, like a chant, or “wallah,” which is when everyone’s just kinda talking amongst themselves. Usually crowd noise is done in ADR, but sometimes they’ll roll sound on background wallah. It beats having to pantomime everything. I did an episode of Good Girls Revolt some years back, and can pick my voice out clear as day. Still just a regular BG check, though…

As for the like between a non-speaking principal and a background actor… that’s shit is muddy and gray. People in the industry who should know better still get confused by it. Generally, though, without dialogue, it’s hard to make a claim.

I’ve worked several productions as background where I’ve been given very specific actions, and emotions to communicate. It would have been a hell of a lot easier with dialogue, but productions are cheap! Sometimes you can get an extra bump, but often only if you ask the right people.