Daniel Radcliffe, who acted opposite Rickman in all eight Harry Potter films, also paid tribute.

"Alan Rickman is undoubtedly one of the greatest actors I will ever work with," Radcliffe wrote on Google+. "He is also, one of the loyalest and most supportive people I’ve ever met in the film industry. He was so encouraging of me both on set and in the years post-Potter. I’m pretty sure he came and saw everything I ever did on stage both in London and New York. He didn’t have to do that. I know other people who’ve been friends with him for much much longer than I have and they all say 'if you call Alan, it doesn’t matter where in the world he is or how busy he is with what he’s doing, he’ll get back to you within a day.'

"People create perceptions of actors based on the parts they played so it might surprise some people to learn that contrary to some of the sterner (or downright scary) characters he played, Alan was extremely kind, generous, self-deprecating, and funny. And certain things obviously became even funnier when delivered in his unmistakable double-bass.

"As an actor he was one of the first of the adults on Potter to treat me like a peer rather than a child. Working with him at such a formative age was incredibly important and I will carry the lessons he taught me for the rest of my life and career. Film sets and theater stages are all far poorer for the loss of this great actor and man."

Emma Watson shared this note on Facebook.

"I'm very sad to hear about Alan today," she wrote. "I feel so lucky to have worked and spent time with such a special man and actor. I'll really miss our conversations. RIP Alan. We love you."

Emma Thompson, who frequently worked with Rickman, penned this tribute.

"Alan was my friend and so this is hard to write because I have just kissed him goodbye," she shared in a statement.

"What I remember most in this moment of painful leave-taking is his humor, intelligence, wisdom, and kindness. His capacity to fell you with a look or lift you with a word. The intransigence which made him the great artist he was — his ineffable and cynical wit, the clarity with which he saw most things, including me, and the fact that he never spared me the view. I learned a lot from him.

"He was the finest of actors and directors. I couldn’t wait to see what he was going to do with his face next. I consider myself hugely privileged to have worked with him so many times and to have been directed by him.

"He was the ultimate ally. In life, art, and politics. I trusted him absolutely. He was, above all things, a rare and unique human being and we shall not see his like again."