This is a great question. There are many techniques that Zemeckis uses. Let me name a few them.
First, Zemeckis uses something called Motion capture (mocap), which seeks to capture the motion of actors and using that information to animate digital characters in 2D or 3D. In this case, it was 3D. By doing this, there is more that you can do with animation. It can be simultaneously realistic and fantastic. This technique can be very powerful in movies like, A Christmas Carol. For instance, through this technique Scrooge can be portrayed in exaggerated ways and yet still surprisingly be life-like.
Second, one of the most remarkable techniques is that Jim Carey actually played several roles. He played four roles: Ebenezer Scrooge, ghost of Christmas past, ghost of Christmas present, and ghost of Christmas yet to come. By doing this, there is a subtle connection between characters, which can add to character development in interesting ways.