A weaker Haag, Narnhofer and Stein (HNS) prescription and a weaker Hessling quantum equivalence principle for the behaviour of thermal Wightman functions on an event horizon are analysed in the case of an extremal Reissner - Nordström black hole in the limit of large mass. In order to avoid the degeneracy of the metric in the stationary coordinates on the horizon, a method is introduced which employs the invariant length of geodesics which pass the horizon. First the method is checked for a massless scalar field on the event horizon of the Rindler wedge, extending the original procedure of Haag et al onto the whole horizon and recovering the same results found by Hessling. The HNS prescription and Hessling's prescription for a massless scalar field are then analysed on the whole horizon of an extremal Reissner - Nordström black hole in the limit of large mass. It is proved that the weak form of the HNS prescription is satisfied for all finite values of the temperature of the KMS states, i.e. this principle does not determine any Hawking temperature. It is found that the Reissner - Nordström vacuum, i.e. T = 0, does satisfy the weak HNS prescription and it is the only state which also satisfies Hessling's weak prescription. Finally, it is suggested that all the previously obtained results should still be valid if the requirements of a massless field and of a large-mass black hole are also dropped.