I think that the best actors are able to trick themselves into becoming emotional when they say their lines. Bad actors aren't convincing because they do not have the emotional thrust to come across as genuine. When we speak freely, the words that we string together into sentences have a neurological pressure behind them. Anytime that we are motivated to do something, dopamine surging from lower brain centers helps to guide our diction and compose our nonverbals. On a neuroscientific level, heartfelt speech has momentous subliminal force behind it. If you are only using your frontal cortex to guide speech, it will not have this quality and will appear operantly counterfeit to anyone with a trained ear.
Sometimes I will say a line that I like to little effect and wonder why it fell flat. I think that to recreate great lines effectively, you can't just try to say the line the way you have heard a good actor say it. You actually have to be emotionally impelled by the words, you have to feel motivationally compelled to say them, as if, at that moment, there is nothing else you would rather say. Reward systems such as the mesolimbic and mesocortical dopamine pathways have to "think" that they are accomplishing something immediately useful for them to create the panoply of unconscious accentuations. When you act genuinely in this way, aspects of the delivery (such as the intonation, inflection and affect) emerge reflexively and unconsciously. I think that this is why it is hard to successfully and persuasively recreate another person's mannerisms, unless you are able to recreate their motives.
Acting involves coercing emotional centers of the brain to cooperate. These limbic regions were built to be turned on and off by “hot” emotional centers, not by our “cool,” conscious mind. They were never evolutionarily programmed to aid humans to manufacture premeditated action in drama or film. We have to recruit these regions deceptively - they have to believe the lines, even if we don't. Impulsive or opinionated people say exactly what they are thinking in front of others. Inhibited people, on the other hand, can come across as inauthentic because they often may not really mean what they say. They know what they are supposed to say, and they say it, but to most people it will sound like what it is - emotionally contrived and devoid of the spontaneity and sentiment that should accompany it. All my life, I have struggled to do a convincing job of being myself and I have found that the only way to do this is to premeditate less, extemporize more, and, quite simply, do what I want more often. This may be a dangerous mode of operation for a young child because a child's first reactions may be morally inexperienced or lacking in social graces. After a full childhood of experience with how to act, it seems that my disinhibited inclinations are finally socially permissible so perhaps I should finally give myself permission to operate off-the-cuff with less deliberation and more theatrics.