Non satis est pulchra esse poemata; dulcia sunto
Et quocumque volent animum auditoris agunto.
Ut ridentibus adrident, ita flentibus adflent

humani vultus. Si vis me flere, dolendum est

primum ipsi tibi: tunc tua me infortunia laedent,

Telephe vel Peleu; male si mandata loqueris,

aut dormitabo aut ridebo: tristia maestum
vultum verba decent; iratum, plena minarum
ludentum lasciva, severum seria dictu.

It is not enough for poems to have beauty; they must be sweet, tender, affecting
and able to lead the soul of the hearer where they want.
As we grin among those who are smiling, 
so we tend to well up around those who weep. 
If you want me to cry, you yourself must first feel anguish 
Then your misfortunes will move me, O Peleus or Telephus;
if you speak inappropriately, I will doze off or laugh out loud:
sad words require a mournful expression, 
angry ones need a face full of menace, 
Naughty words suit a playful mood, 
serious words go with sober topics.  

- Horace, Ars Poetica