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1.
When an actor is able to work more professionally than his/her scene partner--i.e. more like Matt Damon than Ben Affleck. If an actor were to step on his/her partner's lines or self-indulgently ad-lib in order to try to steal the spotlight, he/she would be considered the "Affleck" of the scene. Assuming the 'Affleck's partner is professional enough to at least learn the lines, so meticulously assembled by the screenwriter; and not mug or over-emote like he were playing some pimp in a JLo video, he would be considered the "Damon." The saying would be employed in the case that the scene partners were in contention as to which were the comparative "Affleck" of the scene.
Kevin Smith:

"IF YOU(Jason Mews, portraying Neo in a scene from "The Matrix")'RE GOING TO AD LIB, DON'T USE THE WORD "PENIS"!
Erica(his scene partner who had previously stepped on Jason's line): "WHO'S THE AFFLECK NOW?"
by microsleeper October 12, 2011