'This Is Us': M. Night Shyamalan and 9 More Stars Join Season 4 in Mystery Roles

NBC's family drama dropped a big casting surprise on Saturday.

The Pearsons are expanding.

This Is Us dropped a huge season four surprise on Saturday, which happens to be the Big 3's birthday, revealing a dozen new additions to NBC's hit family drama -- and six familiar faces who will be returning. 

The Sixth Sense director M. Night Shyamalan, who has appeared in front of the camera before in bit parts, is the most substantial name to join the series, but in true This Is Us fashion, character details are being kept under wraps for his role and all other new additions.

Omar Epps, Timothy Omundson (Psych), Nick Wechsler (Revenge), When They See Us stars Asante Blackk and Marsha Stephanie Blake, Bahara Golestani, Julian Silva and Auden Thornton round out the new cast members.

Once Upon a Time star Jennifer Morrison, who will have a "substantial presence" in the new season (and can be seen in the trailer wearing army fatigues), was previously announced. 

Additionally, returning guest stars include Griffin Dunne (Nick "Nicky" Pearson), Tim Jo (Randall's campaign manager Jae-won Yoo), Ron Cephas Jones (William), Elizabeth Perkins (Janet), Phylicia Rashad (Beth's mother Carol) and Caitlin Thompson (Kate's friend Madison). 

While we wait for the mysteries of these new characters to unfold, you can watch the season four trailer below for clues.

The casting reveal comes after nearly two weeks of lead-up on social media pointing to Saturday's anticipated weekend drop, after creator Dan Fogelman revealed earlier this month at a This Is Us pancake breakfast that "there are going to be a lot of secret new cast members who are going to be a big deal coming up."

"In the series, we have what I call little markers, little stakes we put in the ground of these episodes that serve as grounding points for the whole arc and the whole series," he said. "Our first two episodes of this season are two of those and it's a big, bold, potentially polarizing -- but in the best possible way -- start to the season."

Fogelman's remarks elaborate off comments he made during an Emmy For Your Consideration event in June, where he teased that fans may be in for a ride when the season four premiere hits the small screen. 

"It's a really ambitious season," Fogelman said. "We're in the middle of these characters' journeys and that's a really exciting place to be. I can't tell [you] much about the storyline, but I think the first episode... people will be talking about it. It's different and unusual."

"Our first episode, our universe fundamentally expands in a way that people might not be expecting," Mandy Moore hinted to ET's Keltie Knight in August. Added co-star Justin Hartley, "Some new characters being introduced, some new relationships are going to be evolving."

Last time we left the Pearson family, the season three finale raised several looming questions -- from the mystery of Rebecca's well-being to how Jack's younger brother, Nicky, comes back into the family's lives to the unclear fate of Kate in the future.

NBC rewarded This Is Us with a huge three-season order in May, ensuring its place on the schedule through at least 2022. 

Created by Fogelman, This Is Us centers on the Pearsons and their loved ones at various points of the family's history. Led by Milo Ventimiglia, Moore, Sterling K. Brown, Hartley, Chrissy Metz, Chris Sullivan and Susan Kelechi Watson, the hour-long family drama debuted in 2016 to near universal acclaim and has become an awards darling, collecting Emmy, SAG Awards and AFI Program of the Year honors.

This Is Us is nominated for nine Emmys, including Outstanding Drama Series, and individual acting nominations for seven of its actors: Ventimiglia, Moore, Brown and Sullivan, as well as guest stars Michael Angarano, Jones and Rashad.

This Is Us returns Tuesday, Sept. 24 at 9 p.m. ET/PT on NBC, with an extended premiere.

To stay up to date on breaking TV news, sign up for ET's daily newsletter.