The show was created by TV icon David E. Kelly, who brought us classic shows such as Boston Legal, Ally McBeal and Chicago Hope. Kelly based this show on a real-life advertising exec, who Williams gets to play.
When ET visited the set, the cast and crew were shooting a scene in which Gellar's character Sydney has come up with a promotional idea involving a giant cup of coffee, and as you would expect, things go horribly awry when the coffee begins raining down on everyone like a hot storm.
"This is based on a real promotional thing they did in Times Square, in New York," William's told ET's Brooke Anderson. "The idea that this is real is kind of wonderful."
What must really be wonderful is getting to act alongside a pillar of comedy like Williams. The question is: would it be too daunting of a challenge for the other members of the cast?
"The thing that people don't realize about Robin Williams is -- there he is, top ten greatest comedians of all time at standup -- but he's also an Academy Award winning, Julliard-trained actor," said Gellar, talking about why Williams is such a powerhouse. "There's nobody else ever that fits that bill."
James Wolk said that working with Williams was just really exciting. "It doesn't feel intimidating at all when you're on set with him because he's a really generous, caring guy, and an amazingly generous actor. I think the only time I had that kind of intimidation of him was the first time I met him, because I had grown up watching all of his movies."
Check out the video to see more from the incomparably hilarious Robin Williams, and hear from other stars of The Crazy Ones about what it's like to work on the new series.