You're right, that was a very realistic slap. I'll have to re-watch the scene now, but I'd be willing to bet Vestoff and Selby agreed that he would really slap her. (One actress who has insisted on truly being hit is Ann-Margret, as in her harrowing performance in "A Streetcar Named Desire.")
As much as I abhore violence, domestic or otherwise, I have to say my thoughts on viewing this effective scene with Quentin and Samantha were "she deserves it." Of course, hitting someone is wrong, but Samantha's unrelentless taunting of Quentin about Tad almost make that slap justified. (And he didn't continue to beat her, so it was just one push and one slap, which were not unprovoked --
[spoiler]especially when we later find out she's lying about it anyway.[/spoiler]
Someone in another thread recently opined that the writers had changed Samantha's character from the original conception of the characer.
I disagree with that assessment. There have been a couple of hints along the way -- by way of statements other characters have made -- that Samanthat is devious and cunning ... we just haven't seen evidence of that until now. To this I would give credit to the writers/concept people for writing Samantha as a real, layered character, someone whom it takes a while for us to really know. And we'll continue to learn more about her character as the storyline continues.
I would contrast this with the very abrupt and unjustified re-writing of Beth's character in the 1897 storyline.