Being that I have been an extra and been on movie and TV sets I can tell you but wont ruin it for you.lol
There is very specific thing that is done about background conversation and they have a name for it but can't remember what it is called at the moment.When I remember I will let you know. It is actually scripted to every detail and rarely are they using real words or sounds if they are close to the speaking talent at all. It depends on many things like location of mics and the set up of the shot. Some extras have a real part with specific things to say or do and some are more for background.Even things like when someone walks by, that is planed out perfectly. Remember they have to shoot the same thing several times from different angles and then piece it together. If it isn't done the same they wont be able to piece it together. I'm sure everyone has caught the mistakes in movies and TV when it happens. We did a commercial where we all had to ice-skate in the background. My kids were so sick of skating the same pattern for hours I thought they were going to loose their minds.They didn't complain when they got the check and it was the money for a vacation we all wanted to go on.lol They have been in enough things to know how boring all these things really are.Between stuff they have done for me, my family and friends they have been in front of the camera since they have been infants.My daughter hates it, my son's like the money. They all prefer working on the other side of the camera or in the edit suite.

LOL If you tell them what they are really seeing in the food shots it will disappoint people.I'll give you one little trick that is used a lot .Many times when they need milk to look good or more to the point they want milk to be in a certain place and stay there they use Elmer's glue.Didn't you ever wonder why the Wheaties never just soak in the milk and get soggy when you see those close ups? It just sits on the flakes.They also didn't use milk plain for the "Got Milk" campaign to get all those people to have a milk mustache. Milk just wasn't thick enough.As for ice cream here is something to think about. How do they get ice cream under all those hot lights yet it doesn't melt. hhhmmmm..... There is a secret there as well.There are times you want things to shine , others to be dull ans sometimes to steam. Did you ever try to video steam? There are tricks to all of that and people who specialize in food.