What is a key characteristic of someone who is "mercurial"?

The term "mercurial" is derived from the planet Mercury, which is known for its rapid movement. In a figurative sense, describing someone as mercurial means they exhibit quick and unpredictable changes in mood, behavior, or temperament. This quality can manifest in emotional volatility, where an individual may shift from one feeling or state to another without warning. Such variability makes the individual hard to predict or rely upon in terms of emotions or responses.

In contrast, the other options reflect characteristics that are stable and consistent. Someone who is consistent and reliable, or steadfast and unwavering, embodies traits of dependability and predictability. Similarly, being predictably calm indicates a steady and even disposition. These traits are diametrically opposed to the idea of being mercurial, highlighting the nature of emotional instability and frequent change that defines a mercurial individual.