Take someone or something at face value
To take someone at face value is to judge him from superficial appearance.
Don’t take my words at face value, for I may be wrong.
Fair-weather friend (A person who is friendly only when you are rich and happy)
Like everybody else I have had my ups and downs in life and the person I hate most is a fair-weather friend.
Fall flat (fail)
My plan to buy an apartment fell flat as I failed to get a home loan.
Fall from grace (= lose the affection of a superior, fall from a position of high esteem)
His fall from grace was all the more shocking because nobody was expecting it.
Fall head over heels in love (Be madly in love with somebody)
While he was at university he fell head over heels in love with a girl who was his senior.
Far and wide
His fame as a writer spread far and wide. (= He became very famous as a writer.)
Far-fetched (unlikely to be a fact)
His claim of being a great orator must be far-fetched because he is a person who hardly opens his mouth in the presence of strangers.