From my experience in the US (having grown up there):
Usually, in restaurants, people order their own appetizers and eat them for themselves. But often, if everyone at the table agrees (key point), we order a number of appetizers (or “combo” or “sampler” appetizers) to share, thus getting a sampling of different kinds of foods.
During the main course (which we in the US call the “entree” for some reason) we generally order for ourselves and eat our own dishes. Sometimes we might say to someone near us, “Would you like to try some of mine?”, but the more formal the event, the more we keep the food to ourselves.
Food taken from a common serving plate is transferred to your own plate by means of a serving spoon or some other utensil people don’t put in their mouths.
It’s common for couples to try food from each other’s plates, but only if someone offers their food or asks the other person. It’s rude to simply grab something off the other person’s plate without consent, or to put something on the other person’s plate for that matter.
That being said, it’s common in Chinese (especially Dim Sum), Thai and other Asian restaurants, as well as restaurants that serve meals “family style” (like some German restaurants), or Tapas, that we order food for everyone and everyone shares, as if you’re at home with your family.
Now, here’s one of my pet peeves eating with Chinese people here in Taiwan: it’s when someone takes it upon himself to grab something I don’t want (peeve #1) with his own saliva-covered chopsticks (peeve #2) and plonk it into MY bowl (peeve #3). “I’m sorry, sir, but there’s a reason I didn’t help myself to that fermented squid eyeball. And there’s also a reason that hepatitis B is so prevalent here.”
Another pet peeve here in Taiwan is the lack of two things I consider necessities when eating: napkins and drinking water. I try to make sure I bring my own…but then I’m expected to share!!