How does fast food restaurants make grilled chicken
If you are wondering explicitly about Nandos, it is marinated for 24 hours before its cooked. It is then cooked before its ordered: 


  [The chickens] are cooked in a special oven for an average cycle of 30 minutes,
  reaching a temperature of at least 75C, then kept in a warming drawer
  at a temperature of 63C.


Source

When an order is placed, they take the Chicken and finish it on the grill.