Imagine this...
You and Cas are going on your first case. When interrogating a suspect, Cas tries to play bad cop.
"Okay so it says she was here on the night of the attack. I say we ask her some questions" you say
"Do you think I can try to ask her some questions?" Cas asks
"Sure Cas"
Castiel walks towards her to ask her a question. You think he is gonna ask if there were any cold spots but instead he slams his hands on the table and asks
"WHY DID YOU KILL YOUR HUSBAND!" he shouts and you quickly pull him away from her
"Cas you don't just yell questions at suspects like that"
"I was trying to be bad cop"
"Well maybe you should try to be a good cop next time. I'll handle it from here" you say patting him on the shoulder
"Okay"