Continuity mistake: The wound and the sleeve of the jacket on Trinity's arm changes after she's cut by one of the twins. When she's cut, the wound and rip in the jacket are thin and diagonal. As Morpheus is calling Link, the wound is not really visible and the cut in the jacket is now larger and squarer, and has changed direction on her arm. (01:21:00 - 01:22:50)
Chosen answer: Orson Wells first acheived this in "Citizen Kane." It's a combination of position between the lens and actors and the lens focus. There is no exact formula on how to acheive it; mostly trail and error. For example, have the lens tighten in on one actor and have the other move around until they come into focus.