wouldn't

butter wouldn't melt in someone's mouth

The identified person appears to be benign, mild-mannered, or calm (but with an integral suggestion that, to the contrary, they really are untrustworthy, disagreeable, or mean-spirited).

wouldn't be seen dead

To find incredibly horrifying; embarrassing or otherwise entirely unsuitable for oneself.

wouldn't shout if a shark bit him

To be frugal or miserly.