Great question. This is one Quentin Tarantino often talks about. While he focuses on genre movies, this tip works in drama as well. The tip: don’t have the characters constantly talking about “the plot”. It becomes too much “hitting it on the nose”. Let the circumstances develop where the main characters speak more naturally or at least seem to.
If you watch the second scene of “Pulp Fiction” where the two hit men, Vincent and Jules, played by John Travolta and Samuel Jackson, are bantering, they never talk about what they’re about to do. They discuss the names of McDonald’s hamburgers sold in Paris. Which completely makes sense. By not having the characters talk about their hits, we realize they’ve been doing their hit jobs for awhile. When they engage in the hit, it’s a kind of zinger because they were just jawing away a few moments earlier on a mundane subject. It makes it more fun. Or more disturbing. Jules even eats part of a hamburger at their hit location!
People who are engaged in the same kind of business for a long time don’t necessarily talk constantly about it. What’s the point? They’ve done it before. And by keeping the dialogue away from the “plot” it makes it seem more realistic and again far more interesting.
If you take another example, “Butch Cassidy and the Sundance Kid”, the characters don’t constantly talk about their “jobs”, meaning their criminal hits. They discuss other things.
Vincent (Travolta) and Jules (Jackson) are on their way to a “hit” but they don’t discuss their mission. They talk about McDonald’s hamburgers in Paris.