That's completely separate from what Jorg is talking about and what I mentioned.

It's called "crossing" when actors are used to cross in the background so it looks busy.

You're talking about holding or doing ADR because of the doors opening/closing on dialogue.
Two completely different things.

Heh as someone who works as an actor on many sets, you never have someone enter thru the door without someone "crossing" in the background.
Happens in almost every show, movie, etc

I missed out on the AMA!
But here's one question...

Is there any episodes of LD you wish you could have done but couldn't due to budget or time?
Any callbacks?
Any actors/characters you wish you could have brought back? Or sooooo close to bringing back but fell out last minute?

They used it often, very often in the first 2 seasons.
Here's one clear example in season 4