Idiom for someone acting extremely out of character

Here is a common instance of this ....

Maan thinks Bhaskar is acting extremely out of character. So Maan says: "Who are you? And what have you done with Bhaskar?"

Heartspring suggests The Farlex Dictionary of Idioms {at The Free Dictionary} as a reference.

The expression using Kumbhkaran is understandable for anyone familiar with Kumbhkaran, and obscure for anyone not familiar. If you want to adapt the expression to an English audience, you could just pick your favourite character from mythology or books or movies or comic books, or your favourite animal, or your favourite physical certainty, and get an equivalent of the expression.

You could also keep the Kumbhkaran reference, and just add a footnote explaining who Kumbhkaran is. If I was a reader I would prefer this solution. Sadly, publishing houses would probably prefer the solution of adapting the reference to western culture.

• "It was as if Obelix suddenly began preaching non-violence and veganism."
• "It was as if Popeye the sailor suddenly expressed a dislike of canned vegetables"
• "It was as if Sherlock Holmes had become bored of mysteries"
• "It was like hearing Scrooge McDuck complain that his piggy bank was too heavy"
• "Had Zeus become the god of calm weather, abdicated his throne and remained faithful to his wife?"
• "What next? Do frogs complain when the weather is too humid?"
• "It was as if birds decided not to fly and fish not to swim"
• "It was like hearing my cat proclaim a taste for lettuce leaves and a dislike of perfectly good fish"
• "It was as though sloths became frantic and leopards became slow"
• "It was as if wood was tired of floating and rocks complained about sinking"
• "It was as if copper had enough of conducting electricity"
• "It was as if gravity decided to stop making apples fall"
• "It was as if the sun was tired of shining and the moon of reflecting its light"
• etc.

• It was as if Dr Jekyll had metamorphosed into Mr Hyde. Or perhaps vice versa.

[from the famous story by Robert Louis Stevenson; Wikipedia]

(American) English doesn't really have an idiom like the Kumbhakarna one. The closest we have is "Who are you and what have you done with X?", as GEdgar shows, but it doesn't really fit.

But that doesn't mean we can't make one up! One possibility is to simply switch "Kumbhakarna" with "Garfield", a cat from a comic strip of the same name by Jim Davis who's pretty much a modern incarnation of Kumbhakarna. It'd be just as weird for him to wake up at dawn and go on a diet.

Other possibilities include "Bart Simpson deciding to study for college and stop pulling pranks", "Dr. Brennan deciding to give up science and become a priest" and "Zeus deciding to bag it up and stay faithful to his wife".

However, none of these are idioms, they are metaphors that reference popular culture, and you need to know your audience to be able to execute them well. Since Vikram Seth is himself Indian and the book is set in India, it's possible he's just doing the same thing with this Kumbhakarna reference, but instead using it to indicate what kind of person Maan is: a proud Indian who knows his culture well enough to make a reference like this.