I discovered a way of doing this almost perfectly everytime without that stupid bitch shooting you.
I was having the same trouble, walking forward, no dialog prompt and then shot.
Basically have the cursor lined up over Brenda and the hostage. No need to edge forward that slowly, but don't rush in. As soon as she says "get back" and points the gun at you, move the cursor with the RS fairly quickly all over the top half of Brenda and the hostage. This I found prompted the dialog box to come up.
It seems in my case that the cursor position is glitchy and may be key in getting the dialog prompt to come up.
Jusy remember to choose the less aggressive statement each time otherwise the bitch shoots you anyway!
100% Retail Completed (360): 161
100% Arcade Completed (360): 115
100% Android Completed: 1
Currently working on:
Retail: Lego Hobbit