With a mortar and pestle, grind together ground peppercorns and garlic. Add cilantro and work into a paste. Stir in soy sauce (or fish sauce) and vegetable oil until evenly mixed. This may, also, be done in a small food processor or food grinder. Keeps up to 4 days.
In a 1-gallon ziplock bag, combine marinade and chicken, pork or fish pieces, messaging to evenly coat; chill 1-2 hours or overnight. Please, don't forget to season with salt before grillling.
For a compatible dipping sauce, see Thai Sweet Chili Sauce.