Sweet-talk head of casting over a part - Juliet, not Romeo (6)
I believe the answer is:
cajole
'sweet-talk' is the definition.
('cajole' can be a synonym of 'sweet-talk')
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for cajole that I've seen before include "Persuade using flattery" , "Coaxingly persuade" , "Coax, try to persuade" , "Inveigle by gentle urging" , "Wheedle - coax" .)