Conversation Interrupted

How do you format dialogue when one character interrupts another? Just use a couple of em dashes.
See how Dennis Lehane does it in Mystic River, page 261.

“Sure. Hell, I think she still loves me.” He stubbed out his cigarette. "She calls me all the time. Calls me and doesn't talk." 

"Wait, she—"

"I know," he said.

"—calls you up and doesn't say a word?"

Don't forget to use an em dash, not an en dash or a hyphen. For the difference, check out this site: http://cutewriting.blogspot.ca/2008/06/en-dash-em-dash-and-hyphen.html#axzz2JfAbcmyO