Cast members of upcoming NBC series “The Slap” — Peter Sarsgaard, Zachary Quinto, Brian Cox and Michael Nouri — looked like they’d been slapped in the face after unintentionally racking up a huge tab at ritzy Madison Avenue restaurant Nello during a lunch break.

The show, which also stars Uma Thurman, was shooting nearby when Nouri brought his mates to Nello, where he’s a regular.

As the group settled in, owner Nello Balan (who’s been called “famous for not disclosing the prices of his specials”) asked, “Why don’t you try our truffles?” Cox obliged. But, according to a spy, the star and his fellow thespians were shocked when the bill for that dish alone was a whopping $500.

However, Balan — who in 2013 sold a 4-pound truffle to a Russian oligarch for $95,000 — told us, “They got the check by accident . . . I ended up buying the food for them.”

He claimed the “small plate of truffles” actually cost $250, and said that Cox joked, “That’s all?”

But a source insisted the actors paid and, “Brian took his truffles to go” back to the set.