Emotional behaviour of cast with their acting (9)
I believe the answer is:
theatrics
'emotional behaviour' is the definition.
(theatrics can mean theatrical behaviour)
'cast with their acting' is the wordplay.
'with' says to put letters next to each other.
'acting' is an anagram indicator (letters become active or change).
'cast' after 'their' is 'theircast'.
'theircast' is an anagram of 'THEATRICS'.
'of' is the link.
(Other definitions for theatrics that I've seen before include "Histrionics" , "dramatic outburst" .)