Okay so as we all learned in episode 3 is Crane broke the mirror and took one piece of it with him. Now Bufkin explains that it could take days for the mirror to regenerate. Now what if the only way to bring back the mirror is to say Bloody Mary's name as the urban legend says.
The mirror will regenerate and no longer be able to hold back secretes. After Bigby gets checked up by Dr. Swinheart, Snow and Bigby go back to the mirror and say something about Bloody Mary. They will question where BM took Crane, and the mirror will magically regenerate reveling the location.
This seems to make sense because in the trailer it shows BM outside with her magnum. Also Snow will start bossing everyone around and accuse everyone she knows.
Snow will question:
Toad & TJ
Basically Snow will question everyone you talked to in episode 1 and new characters. She will make them prime suspects
Snow will use the mirror to track down the suspects and Snow will use force. She will be like Rick from TWD doing things without question. The mirror will try to avoid giving Snow answers, but she threatens to throw him down the well. Mirror gives up and that's why Toad, Bufkin, everyone who's in the trailer is being questioned by Snow.