You can purchase a leg of lamb either boneless or bone-in. A boneless leg is sold either butterflied, or rolled and tied. The butterflied leg is great for grilling without a rotisserie. This cut has a more even thickness and will cook more evenly. Bone-in or rolled and tied, really need to be grilled on a rotisserie to get the necessary evenness and also helps to avoid drying out the meat.
To grill a great leg of lamb, start with a marinade. Since lamb already has a strong flavor, choose a marinade that has plenty of flavors. Lemon, garlic, and herbs like rosemary and cilantro are good flavors to work with. The marinade should also contain enough vinegar to help carry the flavors deep into the meat. For a boneless leg plan on marinating the meat for about 4 to 6 hours, for a bone-in leg try marinating overnight. Let the leg sit at room temperature for 30 to 45 minutes before you put it on the grill.
With the leg marinated, it's time to grill. Since a leg of lamb can weight in at as much as 8 or 9 pounds, you will need to grill it indirectly over a medium fire for nearly the entire cooking time. This will take a few hours depending upon the size of the leg, but it's this slow roasting that will make it flavorful. A bone-in leg will take longer to cook than a boneless leg. Always use a meat thermometer to test for doneness and remember to keep the thermometer in the meatiest part of the leg, away from any bone.
Start by grilling the leg over a medium high, direct heat in order to sear the surface. This will give the meat a crusty surface and add texture. If you are not using a rotisserie, turn every 5 minutes to sear the surface evenly. On the rotisserie you won't have to worry about this. Either way grill over direct heat for about 15 minutes, then move to indirect grilling and cut the temperature down to medium. Watch for flare-ups during the direct grilling phase. The leg of lamb will need about 20 minutes per pound to cook so plan accordingly.
When the middle of the meat reaches a temperature of about 145 degrees, it's time to take the meat off the fire. You want to let a leg of lamb rest for 20 minutes before carving it. This will even out the temperature, make sure it's completely done, and allow the juices to flow back into the meat. Carve the meat, starting from the thick end and work our way down to the shank; making small pieces.