Here are Aamir's words on the double role of Dhoom 3 -
Q: The double role in Dhoom 3 is different from what we are used to
seeing, as there are many close-up shots and movements involving both
Sahir and Samar in the same frame. How was the execution process for
you as an actor?
Aamir: With newer technology it becomes difficult for the actor, as the kind
of shots you can design for a double role are limitless. Earlier due
to constraints of technology, you could not touch each other in a
double role. Physical contact could not be captured, as you would end
up touching yourself on screen. The camera used to be pretty-much
static and in wide. Here were are hugging each other, exchanging hats,
one is giving an Apple to the other…Because with motion control
cameras, you can duplicate the same moment a number of times. The
angle and position are all locked in a computer, so a situation can be
recreated identically, multiple times, for an actor to perform
different shots for his respective character in a double role.
For example take the shot in which Samar is getting ready in front of
a mirror! Sahir is talking to Samar’s reflection in the mirror and
asks him about where he is going. Once he names Aliya (Katrina Kaif’s
character) and confesses his love, Sahir comes face-to-face and covers
the mirror to confront Samar. Later Sahir once-again uncovers the
mirror and tells Samar that Aliya is actually coming for him.
Full article at indiatimes.com.