Learn English – What’s the word or name of someone who “gets sick” just by talking about it but they’re not really sick

There's a CNN article about “smartphone zombies” that implies zombie is appropriate (perhaps in its sense “A person, usually undead, animated by unnatural forces ..., with no soul or will of his/her own”). The article also mentions Stephen King's novel, Cell:

... the protagonist, Clayton Riddell, doesn't turn into a zombie because he doesn't own a cell phone. The story is about an event called "The Pulse" that turns cell phone users into vicious, mindless beings.

A person acting against their perceived tendency is behaving uncharacteristically. When the person's behavior is undesirable we may say they are not themselves. But the given examples are of behavior desirable to the speaker, and a phrase that might be used is what a pleasant surprise! But this phrase has other uses; it's not only for when a person behaves uncharacteristically.

Another phrase that focuses more on the person: If the speaker desires an explanation for the unexpected but desirable behavior, they may get their answer by (humorously and informally) asking the other person Are you feeling ok? The speaker may also humorously refer to the emotions the desired behavior promoted in them with I'm shocked or I'm touched. Some speakers jokingly challenge their senses with Do my eyes [ears, for the phone call] deceive me? or reality with Can it be (true)? Others joke about the identity of the other person with Who are you, and what have you done with Mr. X?

If the person has changed from undesirable behavior to desirable, the speaker might ask Why the change of heart? The speaker may further remark that the change is welcome.

As shown we have many expressions for unexpected behavior in a person, but I can think of no one-word interjection used strictly in this context.