Thurio has had a very difficult time of wooing Silvia since Valentine's banishment, so the Duke solicits Proteus's aid.
Duke: What might we do to make the girl forget
The love of Valentine, and love Sir Thurio?
Proteus: The best way is to slander Valentine
With falsehood, cowardice, and poor descent,
Three things that women highly hold in hate. (29–33)
Proteus himself will be the chief slanderer, since Silvia (described as being "lumpish, heavy, melancholy") is most likely to believe what Valentine's dear friend says. Proteus furthermore advises Sir Thurio to whet her desire "by wailful sonnets" and a "sweet consort" (hired musicians).
Proteus's guile having completely duped the Duke and Sir Thurio, the audience must now be fascinated by the potential depths to which this one-time friend will sink in pursuit of his wild fancy. His proven success in wooing Julia serves him well as consultant to the luckless Sir Thurio:
After your dire-lamenting elegies,
Visit by night your lady's chamber window. . . . (82–83)