While most of us probably can’t picture Roth in the role that Rickman performed, that’s ok because Roth would have played the entire role very differently.
At this point, it’s probably hard to imagine anybody other than Alan Rickman playing the role of Severus Snape in the Harry Potter films.
However, as Tim Roth explained during a Reddit AMA, everything would be very different if he had decided to take the role.
Most of us probably picture the faces of the actors from the movies in our minds while reading the books now.