Sarah Michelle Gellar talks Robin Williams, being a mom

(CBS News) Emmy-award winning Sarah Michelle Gellar is best known for playing Buffy the Vampire Slayer but recently joined with Robin Williams in the new CBS comedy "The Crazy Ones."

More than 15 million viewers tuned in to watch Gellar and Williams in the premiere, making it the number one new program of the season.

Robin Williams talks "The Crazy Ones," life after heart surgery, and divorce

However, Gellar told the "CBS This Morning" co-hosts that she wasn't just basking in success of her new program but rather felt more pressure, saying she understands "it's a marathon, not a sprint."

The actress said that the success of the show is the result of being a "great mix" of funny and heartwarming.

Gellar also gushed about working with Williams and playing his daughter on the new show.

"He's a Juilliard-trained actor, so he takes it very seriously. He comes in learning his lines, and sometimes you have to push him to riff, and then once you get him started on the riffing, you don't know where it's going to go," she said.

Gellar isn't the only one in her family that enjoys being around Williams. Her children get excited to see his likeness around town.

"My children call him Uncle Robin," she said. "They'll point out, 'Look there's Uncle Robin on a light bulb, at a bus stop,' and every once and a while I'll look over and I'm like, 'Oh my goodness, this is Robin Williams,'" she said. "I can't think of anyone else. He is an Academy-award winning, top 10 stand-up comic of all time. Those lines don't cross."

The actress has a 4-year-old daughter, Charlotte, and a 1-year-old son, Rocky, with her husband, actor Freddie Prince Jr.

Sarah Michelle Gellar calls "The Crazy Ones" her most fun role ever

She told the co-hosts that working and being a mother is the hardest thing she's ever done.

"Working with one child was one thing, but working with two is entirely different," she said. "I had both my kids' birthdays, they turned 4 and 1 in a span of 24 hours, and we moved, and we launched the show."

For Sarah Michelle Gellar's full interview, watch the video in the player above.