The 'Night at the Museum' Cast Remembers Robin Williams

Night at the Museum: The Secret of the Tomb‘s stars gathered at the N.Y.C. premiere on Dec. 11 to celebrate the family flick’s release – but they remembered their late costar Robin Williams, too.

Williams reprised his role of President Theodore Roosevelt in the threequel, one of his last completed projects before committing suicide in August.

While the cast was sad Williams couldn’t be with them to introduce the movie, they celebrated his life and performance at the premiere.

“He was an amazing performer – an icon before I met him, then a friend,” costar Ricky Gervais tells PEOPLE. “I haven’t met anyone who has a bad word to say about him. He was just [a] lovely, generous, sweet man.”

Ben Stiller says having him in the cast made the franchise a success.

“I remember when the first movie happened, and Shawn [Levy, the director] said he wanted to ask Robin to play Teddy Roosevelt, and I was like, ‘Oh my God, that’s the most brilliant idea in the world. We can’t do the movie if he doesn’t say yes,’ ” says Stiller. “That’s really how we always felt, so he’s an integral part of this.”

Adds Levy: “It’s sad of course, [but] I’m also excited for people to celebrate this guy and his amazing performance.”

It should come as no surprise that Williams was always making people smile and laugh on set.

“My personal favorite was the joy it gave Robin to make Ben Stiller crack up,” the director recalls. “That was always really cute and sweet.”

And like he was so often, Williams went out of his way on set to make others happy. Costar Skyler Gisondo, 18, nearly missed out on prom season because they were filming. So, Williams and Stiller helped act out a skit the teen actor had written to record and ask his then-girlfriend to the dance. They made the video – and Gisondo got his date.

“I feel so lucky to be able to work with him when I did. I learned things from working with Robin I’ll carry with me for the rest of my life,” Gisondo says. “We all miss him very much A guy like that has an impact on people for the rest of their lives in film.”

For more on Robin Williams’s legacy and Night at the Museum, pick up the new issue of PEOPLE, on newsstands now

• Reporting by SARA HAMMEL