"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.
"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," he continued.
"I'm very sad to hear about Alan today," Watson 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."
Read other celebrity reactions to Rickman's death below:
There are no words to express how shocked and devastated I am to hear of Alan Rickman's death. He was a magnificent actor & a wonderful man.
I do not want my heroes to die! Alan Rickman is dead & he was another hero. Alan - thank you for being with us. We are sorry you had to go
I was at Leavesden Studios today when I heard the news. As I walked through the canteen I thought of Alan queuing up for his lunch with us mere mortals. I recalled the trailer in which he offered me some of the greatest advice I ever received about this mad profession we shared. Being back in those corridors made me remember a lot of things and I will treasure those memories all my life. He inspired my career more than he ever knew and I'll miss him.
Long Live Snape. And Hans Gruber. And Sheriff of Nottingham. #saddestweek