"Alan Rickman is undoubtedly one of the greatest actors I will ever work with," Radcliffe wrote. "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."
"Alan was extremely kind, generous, self-deprecating and funny," he wrote. "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 theatre stages are all far poorer for the loss of this great actor and man."
"I'm very sad to hear about Alan today," Watson added. "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."
Radcliffe and Watson co-starred with Rickman in all eight Harry Potter movies, with Radcliffe as Harry Potter, Watson as Hermione Granger and Rickman as Severus Snape. Kate Winslet, who starred with Rickman in Sense and Sensibility and A Little Chaos, also paid tribute to the late actor
"We are all so devastated to lose Alan," the actress said. "He was loved enormously by so many. He was an exceptionally warm and giving man and an utterly phenomenal actor and gifted director. I remember being so intimidated by him when we worked together [on Sense] because he had such a powerful and commanding presence ... But the reality of course, was that he was the kindest and best of men."
In addition to Harry Potter, Rickman was known for Die Hard, Robin Hood: Prince of Thieves and Truly, Madly, Deeply. Harry Potter author J.K. Rowling called the late star a "magnificent actor" and "wonderful man" following news of his death.