Dic: play on

[PHRASAL VERB] V P n
If you play on someone's fears, weaknesses, or faults, you deliberately use them in order to persuade that person to do something, or to achieve what you want. (= exploit)

• an election campaign which plays on the population's fear of change.

.... Cobuild

CF. play (up)on something

1. Lit. to make music on a musical instrument. (Upon is formal and less commonly used than on.)

• Can you play upon this instrument, or only the one you are holding? 
• I can't play on this! It's broken.

2. Lit. to play a game on a field or court.

• Shall we play on the floor or on the table? 
• Let's play on the field. It's dry enough now.

3. Fig. to exploit something—including a word—for some purpose; to develop something for some purpose. (Upon is formal and less commonly used than on.)

• You are just playing on words! 
• You are playing on a misunderstanding.

4. Fig. [for light] to sparkle on something.

• The reflections of the candles played on the surface of the wall.
•  The lights played on the crystal goblets.

... http://idioms.thefreedictionary.com/play+on