I like Gilliam van der Gouwen’s take. I sure as heck would look startled!
It’s crystal clear from the text and the context that Zechariah was alone with the Angel, who all those other people so many artists added? (The local PTB? Other Biblical figures out of time and space? Or?)
In retrospect, had the subject occurred to me, I should have expected it, human nature being what it is: fallible, malicious, self-serving....