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Good actors, especially when they know their character, will come in and either tell you in advance that they have an idea, or in the middle of the rehearsal or the scene they'll let it loose and you go, "Ah that's great."

"Q&A with 'Rescue Me' writer-actor Denis Leary". Interview with Stuart Levine, variety.com. September 7, 2011.
Good actors, especially when they know their character, will come in and either tell you in advance that they have an idea, or in the middle of the rehearsal or the scene they'll let it loose and you go, Ah that's great.