The prosecution’s star witness began testifying Tuesday afternoon.  Amy and T.J. go over what the 25 year old au pair told jurors, giving a graphic and detailed version of the planning and execution of the murders of Brendan Banfield’s wife, Christine and an unsuspecting stranger, Joseph Ryan.   Juliana Peres  Magalhaes  is key to the prosecution’s case, but there are sure to be fireworks later today when the defense begins its cross examination of Magalhaes.