One word to say to someone who's being in a state they're usually not (or doing something they never did)?

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.

Somewhat similar to dingo_dan's uncharacteristically, the phrase out of character is often used to mean

not typical of the apparent character of a person or thing

I cannot think of one word that would fit, but a short expression that may fit in certain situtations is What a surprise! If you are speaking narratively, you can use To my surprise...

Google Translate gives an additional multi-word suggestion, Oddly enough...

I know this question is old, but it was promoted to me and I think it's worth commenting that English has an idiom for this: turn over a new leaf

Mr. X, you've turned over a new leaf!