Acting is very difficult to do well, but at the same time the best acting must look effortless. The simplicity, for me, is simply a matter of accepting that I am this person. I don't believe in seeing the character as different from myself, because for all intents and purposes I am that person. If the audience or camera catches you forcing it, it's not comfortable or pleasant to watch. If you accept that you are the person, then the experience that you bring to it will be full and natural and real, and not forced.
Light stuff is more about timing, focus, attention, and clarity.
Not clarity in your own work, but clarity of the entire scene. Really just listening to the other people and reacting to what they're doing. One shouldn't be doing a very heavy piece during the day and find it hard to have dinner that evening and enjoy themselves. I don't know anybody who doesn't. The best are the ones who deal with that nervousness in the most healthy manner.