They are fixed to whatever supports them.
Thus, animation studios starting with Disney began the practice in the 1930s of maintaining story departments where develop every single scene through , then handing the film over to the animators only after the production team is satisfied that all the scenes make sense as a whole.
Although relatively few titles became as successful as Disney's features, other countries developed their own animation industries that produced both short and feature theatrical animations in a wide variety of styles, relatively often including and techniques.
New York: Three Rivers Press.