After Oscar winning actor Robin Williams died on Monday from an alleged suicide, celebrities have expressed their grief, condolences and thoughts on the tragedy, one of them including Williams' former co-star Sarah Michelle Gellar.

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Gellar, who starred with Williams on The Crazy Ones, which got cancelled last spring, took to her Twitter page to reveal some enjoyable moments she shared with the actor. She posted an array of photos that show her taking selfies with Williams while filming the CBS series. A couple pictures show them in character while others show them behind the scenes, laughing and having a good time.

The actress did not leave a caption in the tweet, but simply left it with happy moments.

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A wide number of other celebrities expressed their thoughts on Twitter, including Jon Favreau, Ashley Tisdale, Charisma Carpenter, Dane Cook and Jason Priestly.

Glee star Lea Michele wrote, "So heartbreaking to hear the terribly sad news about the amazing Robin Williams, thank you for bringing so much laughter and joy to us all."

House of Cards star Kevin Spacey wrote, "Robin Williams made the world laugh & think. I will remember & honor that. A great man, artist and friend. I will miss him beyond measure."

The Vampire Diaires star Paul Wesley wrote, "My deepest condolences to the family of Robin Williams."

Steve Martin wrote, "I could not be more stunned by the loss of Robin Williams, mensch, geart talent, acting partner, genuine soul."

Larry King wrote, "Sorry to learn of the tragic death of Robin Williams. Spent a lot of time with him. Even performed with him in San Francisco. So sad."

Tags: robin williams Sarah Michelle Gellar