The following is from Vikram Seth's A Suitable Boy:
'Do you want some sums?’
‘Not just at the moment,’ said Bhaskar. ‘My head is full of them.’
Maan could hardly believe this response. It was as if Kumbhkaran had decided to wake up at dawn and go on a diet.
For context, Bhaskar is Maan's nephew and loves to do math problems; his refusal here is uncharacteristic. The line as if Kumbhkaran had decided to wake up at dawn and go on a diet is a reference to a figure in the Ramayana, Kumbhakarna, who was famously slumberous and gluttonous. It would have been very out of character for him were he to wake up at dawn and go on a diet. I'm not actually sure if this is a translation of some sort of an Indian proverb or whether Seth is just making an allusion, but either way, this idiom doesn't work in English, because the average English speaker is unlikely to understand the reference.
So, is there an English idiom that can describe someone acting out of character? For an example sentence, it should just be capable of replacing the original line that prompted this question.
(I'm aware that dictionaries list acting out character as an idiom, but I'm looking for something more colorful/evocative/allusory)
To clarify, I'm looking for an idiom to describe someone who's deviating from their normal way of acting, not from society's expectations.