If starting with corn on the cob, cook or grill the corn until just tender, then let it cool slightly. Stand each ear on its flat end and use a sharp knife to cut the kernels off in a downward motion.
Combine the corn, tomatillos, bell pepper, onion, cilantro, and chile pepper in a mixing bowl.
Stir in the cumin and lime juice, then season with salt to taste.
Cover and refrigerate for at least 30 minutes to let the flavors meld before serving. This salsa keeps well, so it's fine to make it further ahead.