SNM:
blimp: The most unlikely things do happen, I don't think suspending disbelief should be a problem. Imagination is what any writer needs!! Predictabilty is definitely the enemy!
I'm going to have to strongly disagree with this. People acting out of character, or the story ignoring its own premises, can bring suspension of disbelief crashing down faster than the stock market in 1929.
Well, AFAIC there's a difference in plot elements being unpredictable--the mother who supposedly went to her garden club slipping back home on foot to catch her kids smoking, for example--and a character's behavior being unpredictable--a girl who finds spankings purely punitive suddenly trying to provoke her babysitter into disciplining her OTK, for example.
Unless there's a solid explanation for a child whom it's been established genuinely dislikes being spanked 'reversing her field' and intentionally earning a spanking, the author is going to lose me at that point. OTOH, Mom stealthily entering the kitchen through the back door while Billy and Susie are puffing away at the table, that's simply an unexpected but hardly implausible turn of events.
My stories frequently use the same characters in different parts of their lives, so I feel justified in having the characters and their interrelationships gradually evolve over years and decades, but AFAIC that's quite different from having a particular character act in opposition to his/her established personality and value system within one specific story--unless a plausible reason is provided, of course...

--C.K.