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In fact, Williams was the reason Gellar wanted to do the show in the first place, telling E! Online at the time: "I stalked him ... I even called his best friend and I was like, 'I have to be on this show. Please tell Robin.'"
"He is a legend," the actress continued. "Think about it. Greatest stand-up comedian of all time, Academy Award winner, and nicest man on the planet."
The two became extremely close during filming, with Gellar saying that her children even called him "Uncle Robin."
After news broke of Williams' unexpected death on Monday, Gellar posted this photo collage of the former co-stars:
On Tuesday, Sarah Michelle Gellar released a more lengthy statement about her time with Williams - "the father I had always dreamed of having" - and the tremendous loss she now feels:
"My life is a better place because I knew Robin Williams. To my children he was Uncle Robin, to everyone he worked with, he was the best boss anyone had ever known, and to me he was not just an inspiration but he was the father I had always dreamed of having.
There are not enough adjectives to describe the light he was, to anyone that ever had the pleasure to meet him. I will miss him everyday, but I know the memory of him will live on."
And to his family, I thank them for letting us know him and seeing the joy they brought him. Us crazy ones love you.
God, grant me the serenity to accept the things I cannot change, the courage to change the things I can, and the wisdom to know the difference."