Final answer:
Stock characters in commedia dell'arte are fixed and stereotypical, representing consistent societal roles or personality types that are stable and not easily changed, despite the use of improvised dialogue.
Step-by-step explanation:
The true characteristic of stock characters in commedia dell'arte is that they are fixed and stereotypical. These characters represent archetypes with specific traits and behaviors that audiences easily recognize and understand. Such characters are not highly individualized; rather, they embody the essence of certain societal roles or personality types, which remain consistent and predictable throughout the performance. The use of improvised dialogue within commedia dell'arte is a distinguishing feature, allowing actors to adapt lines to the situation or audience reactions, but this does not change the fundamental nature of the stock characters themselves, which are long term, stable, and not easily changed. Characters like Arlechino (the crafty servant) and Pantalone (the greedy old man) are iconic examples that have influenced comedic writing in Europe, and have been referenced by playwrights like Shakespeare and Molière.