A good example. And you'll notice they never cross The Lines between Sly and Ahnuld and Ahnuld and Bruce. So what you're seeing here is a great example of The Line in action. You always know who is looking at whom.
Camera angles are ultimately a creative choice, but I have my own golden rule that goes beyond "the line".
A camera angle represents the audience's point of view. Ultimately film is a voyeuristic experience. So the optimal camera angle would be where you'd want to focus your attention if you were wandering around the set, with the added ability to fly or pop from one place to another. This also helps determine how much of a closeup to apply.
There are a million reasons to deviate from this, but in standard one-on-one dialogue, this works best.
Here is a real-world example that everyone can relate to. If you are sitting at a table with a person seated to your left and right, you would probably move your head back and forth to look at the speaker. If you noticed someone getting really upset, you might hold your attention on him or her even when they aren't speaking.
For action scenes, different rules may apply, but for the talky stuff, this seems best to me. When you add a third speaker into the mix, or they begin to move around the room, it makes things more complicated, naturally.
One of the things I don't like in dialogue scenes are medium two-shots where you see both actors in complete profile. It's just not possible to read the full performance from an actor in profile. That's why stage actors always skew their posture towards the audience. Nevertheless, I see these sorts of two-shots quite often, even in films by masters like Hitchcock. There is an economy in this, as you can maybe avoid doing extra takes from individual closeups or over-the-shoulder, but you lose a lot in the process.