The actor was just eleven years old when he was cast opposite Rickman in the Harry Potter franchise, and today voiced his devastation that the actor has died from cancer at the age of 69.
In a lengthy post to his Google Plus page, the star said: "Alan Rickman is undoubtedly one of the greatest actors I will ever work with. He is also, one of the loyalest and most supportive people I've ever met in the film industry."
Daniel, now 26, went on to say that Rickman had been "so encouraging" of him during the Harry Potter years and afterwards, poignantly adding: "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."
The actor went on to say: "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."
Among Rickman's most memorable film roles were bad guy Hans Gruber in Die Hard, the villainous sheriff in Robin Hood: Prince of Thieves and, of course, the intimating Professor Snape in Harry Potter.
Daniel concluded by saying: "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 theatre stages are all far poorer for the loss of this great actor and man."