"Rob wants to have a man to man chat with Rupert to find out exactly what happened between him and Kristen," said a source close to Pattinson.
"Kristen has already betrayed his trust by cheating, so he can't be sure that she will tell him exactly what went on between her and Rupert. Rupert has a wife and a family to save, so Rob feels like he has nothing left to lose and would be honest with him," the source added.
According to the insider, Pattinson feels doubly betrayed because he trusted Sanders as the director of his lady love's film, and had no idea the two were getting so close while he wasn't around.
"Rob really was blindsided by this and he's going through the typical stages of being cheated on," the source said.
"He was hurt and heartbroken, then angry and pissed off, and now he just wants answers. He feels like he needs to know exactly what happened, how far it went and how many times, so he can figure out if this is something he can move past. He's not ready to talk with Kristen, it's too painful, so he's going to Rupert," the source added.
Stewart has apologised and already released a statement calling the affair a "momentary indiscretion". But her cosy pictures with the director have raised fresh doubts in Pattinson's mind.