10 Simple Things Actors Can’t Make Look Convincing
5. Eating
Even though eating is (in theory) one of the simplest acts a person can perform, it can be a nightmare when it comes time for the cameras to start rolling. Since each scene requires multiple takes, wolfing down a burger or taking a bite out of a sandwich causes all sorts of problems. Unless the film crew are monitoring the amount and placement of every piece of food, the continuity isn't going to line up.
One sneaky technique to get around this problem is to keep the food unseen. The Big Bang Theory relied on this method, by regularly having the gang chow down from takeaway pots.
But that's not the only issue. Since actors have to perform each scene several times, they can easily eat too many items on a plate while performing a dinner sequence. To avoid this, and maintain continuity as well as possible, the camera cuts away before a character swallows, allowing them to spit their food out (gross) in between takes. When the camera cuts back to them, they'll pretend to eat, even though there's no food in their mouth.
The scene between Vincent Vega and Mia Wallace in Pulp Fiction perfectly demonstrates this practice.
Despite the fact the pair seem to be tucking into a big meal, neither of them is ever seen swallowing a single morsel of food.