You film Alan Rickman's face while Ellis gives his 'guns...pens...what's the difference' speech offscreen. He gives the 'Hans...Bubby' line. It throws Rickman off for a moment and makes his reaction that little more genuine.

Then you film Ellis giving the speech. Probably after it has been tidied up and reworked a little bit.

Then you edit them together.

I was hoping this was going to answer a question I have asked for a long time but still don't have a clear answer. Is it common to have 2 cameras filming actors simultaneously during a shot/counter-shot scene in a standard Hollywood production, so it's recording their interactions in real time?

Or is it more likely done with one camera, with the actors filmed sequentially and responding to off-camera dialog as they speak their lines. And then the shot/counter-shot are strung together in editing.

Seems to me the one camera would be more logical, as otherwise the lighting resources themselves would have to be doubled and kept out of view. Also I don't ever remember seeing any pictures or footage from a movie set where they have 2 cameras and 2 sets of lights, etc.

The reason I keep asking is that on IMDB in the trivia section you always read some nonsense about somebody's onscreen reaction to some unscripted ad-libbed line being genuine.

Well if they aren't both in the same shot how could it be a genuine reaction if the shot/counter-shot are filmed with one camera at different times? And the dialog may be spoken and recorded hours apart?

Like this scene from the "Die Hard" trivia section:
Hart Bochner's line "Hans... Bubby!" was ad-libbed. Alan Rickman's quizzical reaction was genuine.

They weren't in the same shot, so how can his reaction be genuine when the line may have been ad-libbed several hours earlier or later. If it was ad-libbed at all.

It strikes me as stupid made-up shit that passes for trivia and knowledge on the Internet but wanted to get some opinions on this.

It's amazing that rottenseed says something like that, yet quotes a Charles Bukowski poem on his profile page about comments like this.

I think you're confusing what Bukowski was saying there bubby. Go back and try to read it again.