After being captured, the Joker uses Stephens (the cop watching over him) as a hostage in order to escape from his cell to make his phone call, seen here. Why is there anyone in the cell at all, when they know the Joker is verbally manipulative? It seems purely to be an unnecessary risk.
The novelisation simply states that Stephens is standing guard. No further explanation is offered.
The Joker was certainly damaged — bloody, smeared — but he seemed perfectly content, sitting in the interrogation room, guarded by Gerard Stephens.
“I want my phone call,” he said.
The film script offers little extra by way of explanation, other than that Stephens is guarding the door
INT. INTERROGATION ROOM, MCU, GOTHAM CENTRAL – NIGHT
[The Joker sits, smiling, content. Stephens guards the door.]
Based on the fact that the Joker is a high profile prisoner, that would certainly merit someone keeping a very close eye on him, if only to dissuade other policemen from beating the snot out of him.
He is also in the process of being interrogated about an ongoing hostage/kidnapping situation (hence why he's in the interrogation room). They'd want to have someone in there with him, in case he says something that might be of use. Also, they'd want to prevent anyone not connected with the prisoner from entering the room in case they somehow prejudice his testimony.
First thing, I believe police was not much aware about the Joker being verbally manipulative till that point of the situation. They saw him as someone who can impress local goons and bring them as his gang members. But they underestimated him as someone who could manipulate a straight person like Stephens.
Till that point they thought that other local goons are following him as he is making huge money for them or as someone with more skills with them, which was irrelevant with Stephens.
And they need to have someone stand in the interrogation room with him for safety purposes, in case the Joker could attempt suicide or damage to himself, seeing his previous actions. As he didn't seem to care about his own life.