Christelle asked:

How honest can I be?

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"Honesty is the best policy"! This means absolute honesty but honest comment should never extend
to personal criticism of others which is always unwanted and unnecessary.

There are grades of honesty. You can hint at or imply something which can easily be withdrawn. This
is better than outright honesty in cases of doubt!

What you can be honest about is to an extent socially determined. If someone asks if you think
they're too fat it is not acceptable to honestly say they are because that's not the answer they want.
No-one would dare to say "Goodness, aren't you fat!" Socially, this doesn't apply to thinness but it
should do — it is an unwanted and unnecessary comment.

Rachel Browne