The suspect (2)

As Williams and Susan approached Lance's chamber, they heard things scattering and shattering inside, and they both stopped in front of the door and turned to look at each other.

"You should stay back. I will go in on my own." Williams said to his sister, but she shook her head. 

"No. We should go in together. He won't dare do anything rash if we are together."

Although Williams looked worried about going in there with Susan, he nodded. He knew that she was not going to change her mind, and besides, it was unfair to suddenly make her wait behind when she had come this far. 

While Williams was hesitating to knock on the door, Susan opened it and was about to enter the room when Williams held her back and looked at her with wide, questioning eyes. 

How could she just open the door like that without knocking when she knew she was going into the chamber of a man?