Herbs, like parsley or cilantro, should generally not be diced because dicing bruises them, releasing their oils and diminishing their flavor. Instead, they should be chopped or chiffonaded.
Dicing involves cutting an ingredient into small, uniform cubes. While this is ideal for many vegetables and fruits to ensure even cooking and presentation, it's detrimental to delicate herbs. The force and repeated cuts involved in dicing rupture the cell walls of the herbs, releasing volatile oils that contain their aromatic compounds. This results in a loss of flavor and can even make the herbs taste bitter.
Instead of dicing, herbs should be chopped using a sharp knife with a rocking motion, or chiffonaded (rolled and thinly sliced) for leafy herbs like basil. These methods minimize bruising and preserve the herb's essential oils, maximizing their flavor impact in your dish.
Add fresh herbs towards the end of the cooking process. Prolonged heat exposure can also degrade their flavor and aroma. Adding them at the last minute ensures their bright, fresh taste shines through.