What event transpired between Abigail and John Proctor prior to the beginning of Act 1 of The Crucible?

It is a bit of an understatement to say that Abigail and
John have a shared history prior to the start of Act I.  It is Miller's greatness to
show that some relationships, especially bad ones, between people linger around so that
it is always there no matter what.  Abigail and John are in that position for while much
might have happened since their affair, it is there and this is what ended up
transpiring between both of them.  It is for this reason that Abigail seeks to conjure
up a spell against Elizabeth Proctor.  It is evident from early on that Abigail "will
have her way, whatever it takes to get it," indicative of what she covets that Elizabeth
has:  Her husband.  This intimation given in the first scene is forcefully conveyed in
the second scene, as it is evident that both Abigail and John had an affair at one point
in time.  This is seen in Abigail's attempts to rekindle what was once there, only to be
rejected by John who has moved on from that point.  It is interesting to note that
Miller brings out the idea that personal desire can be the impetus behind social and
political manipulation, something that starts in the first two scenes with Abigail and
will be expanded as the drama unfolds.