In the bottom of a large bowl, whisk together olive oil, garlic, cumin, paprika, and cayenne. Add green beans to bowl and toss to coat evenly.
Place beans in an even layer directly on on a grill pan.
Cook, stirring often, until crisp-tender and unevenly charred and blistered, 10-12 minutes, depending on the amount of char you like.
Transfer beans to a serving bowl and season to taste with salt and freshly ground black pepper. Squeeze lemon juice over if desired. Serve.
As these are often served as a side dish to whatever you're grilling, let the main dish dictate whether you make these with the BBQ lid open or closed. I've made them both ways, just be sure to stir them occasionally either way. And if they're the main attraction, opt for having the lid open so you can keep an eye on them and get the edges charred exactly as you like.