wouldn't you say

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).

do exactly what it says on the tin

To do what is described or what one would expect, with no further explanation needed.

easier said than done

Easy to propose, but difficult to accomplish.

final say

The right to make a final decision.

have a say

To (receive the opportunity to) voice one's opinion.

I see, said the blind man

Expressing confusion.

I'd say

It is my estimate or opinion.

I'll say

Used to indicate emphatic agreement