Is standing by the window muttering about blood something he does all the time?"asked Simon."No,"Jace said. "Sometimes he sits on the couch and does it.

In this quote, Simon is asking Jace if the person they’re talking about stands by the window often and mutters about blood. Jace says that the person sometimes stands by the window but he also sometimes sits on the couch and does the same thing. This implies that muttering about blood is something the person does at different times and not something they do all the time.