Of course not.
Ralph Fiennes for example never read Harry Potter books. And it really shows. His Voldemort is a mere caricature of what Rowling originally envisioned. Voldemort was always a character with generic motivation but at least in books,Voldemort to me always seemed truly frightening. In the films however:
Had Fiennes read the books,he would’ve understood Voldemort’s character a bit more. He wouldn’t make him into a jolly,grinning moron,rather a calculating psychopath. Fiennes can actually play that Voldemort quite well, but it’s obvious that neither the director nor the actor had any intention of actually going deeper with the portrayal.
So no,they don’t have to read the book. But it can be quite useful if they do,since they are bound to have a better understanding of the character and appreciation for the source material.
On the other hand:
Charles Dance never read Song of Ice and Fire and yet did a magnificient job portraying Tywin. So there is no rule. Depends on the script,the director and the actor.