Zachary talked about the scene during a panel at the Television Critics Association winter press tour at the Langham Hotel in Pasadena, California,
The Slap is an eight-episode drama based on an Australian miniseries drawn from local author Christos Tsiolkas's 2008 bestseller.
Zachary was joined at the panel and Q&A by co-stars Uma Thurman, Melissa George - who was also in the Australian version - Thomas Sadoski, Thandie Newton and Peter Sarsgaard plus executive producers Jon Robin Baitz, Lisa Cholodenko and Walter F. Parkes.
Lisa praised child actor Dylan Schombing, saying: "The kid who gets slapped had incredible staying power."
The cast had no doubt that Zachary's character, Harry, was in the wrong for hitting the child, Melissa told the assembled critics.
But Uma made the point that "growing up in the '70s, we got hit all the time. You saw friends getting hit.
"One of the things I felt about this piece is it's a very interesting cultural observation about the changing face of how to be a human being and what's acceptable and what's not."
NBC is due to air the first episode of the drama on February 15 in the US.
Meanwhile, some of the top talent in television attended the Fox Winter TCA in Pasadena, California and posed on the arrival carpet ahead of Q&As on their projects.
Man Men actress January Jones hid her figure with a large beige blazer over a horizontal striped dress that had wide panels of navy blue, bronze, brown and silver.
She completed the look with strappy heels that had bold black and white stripes.
The 37-year-old will star next in the comedy The Last Man On Earth, written by and co-starring Saturday Night Live alum Will Forte.
The sitcom follows Will as the last human on Earth in the year 2022 who searches for human life after the planet's population has been wiped out.
January plays the role of Melissa and will likely be featured in flashbacks when the pilot airs March 1 in the US.
Kerry Russell and her real-life boyfriend Matthew Rhys, who plays her on-screen husband in the FX series The Americans, looked like a happy couple at the event.
The 38-year-old wore a plunging black top, black trousers and white wedge heels.
Her beau wore a dark blue blazer over a crisp white shirt, black trousers and black shoes.
The pair - who play married undercover KGB officers living in Washington, D.C., - have been officially dating since April.
The third season of the drama premieres on January 28 in the US.
Shannyn Sossamon and Carla Gugino will star alongside Matt Dillon, Juliette Lewis, Terrance Howard and Melissa Leo in the midseason-replacement 'event series' Wayward Pines, which is executive produced by M. Night Shyamalan (who directed the pilot).
Matt Dillon stars as Ethan - a U.S. Secret Service agent looking into the disappearance of two federal agents in a small Idaho town.
Shannyn will play Theresa Burke, Ethan's wife, and Carla will take on the role of Kate Hewson, one of the missing agents - who is also Ethan's former lover.
On the arrivals carpet, Shannyn, 36, wore a large light blue dress and chunky grey shoes. And Carla, 43, wore a bold purple dress with a cut-out that put her cleavage on full show. She completed the look with a pair of nude heels.
New Girl star Hannah Simone looked elegant in a light green dress and beige wedge heels.
The beauty reprises her role as fashion model Cecilia 'Cece' Parekh in the ensemble comedy featuring Zooey Deschanel, Jake Johnson, Max Greenfield, Lamorne Morris and Damon Wayans, Jr. in the series, now in its fourth season.
- Daily Mail
