Interacting selectively and non-covalently with an RNA polymerase II (RNAP II) regulatory transcription factor and also with the RNAP II basal transcription machinery in order to increase the frequency, rate or stability of the aggregation, arrangement and bonding together of proteins on RNA polymerase II promoter DNA to form the transcriptional preinitiation complex (PIC). Cofactors generally do not bind DNA, but rather mediate protein-protein interactions between activating transcription factors and the basal RNAP II transcription machinery.