Lean meat such as chicken breast toughens if cooked for too long. The breasts sold as "chicken supremes", usually with a piece of wing bone attached, are thick at the wing end; the heat of a grill chars their outsides quickly, but is slow to invade further. It is worth cutting them into three or four portions. There is a flap of meat that you can slice off; you can then slice off another portion at the rear, thinner end; and slice through the breast at the thick end to produce portions about 50 mm thick.
The amount of flavour imparted to meat by long marinating is often overrated. I usually soak the breasts for about half an hour: perhaps (for two supremes) in the juice of half a lemon, a good glug of olive oil, and a clove of garlic, crushed with a little salt.
You could simply fry the breasts in this marinade, although you would risk burning the garlic. Or you could scrape off the marinade, and cook the chicken on a ridged grill pan or on a dry frying pan. Get the pan hot over a medium heat, and turn the breasts several times as you cook, so that the outsides do not burn before the heat penetrates the centres. They should take about five minutes.