Why is acting not a sin? If lying is a sin then why is acting not a sin? By 'acting' I mean intentionally deceiving others.   

Example: You meet with someone and act happy, but you are mad at them they now are deceived and think you're happy with them.
It cannot be wrong to be discreet or to present a specific outward appearance to others for Jesus commands his disciples that when they fast they are to anoint themselves with oil and they are not to advertise the fact that they are fasting, Matthew 6:16.

Nor can it be wrong to deliberately adopt an appearance that is anonymous or that is unidentifiable for Jesus did so on the Emmaus road, maintaining that presence for quite some time until the two disciples realised to whom they had spoken, Luke 24:15.

But it is clearly wrong to deliberately put on an act in order to present oneself in a good light to others, deliberately enhancing oneself in order to give a false appearance since Jesus condemns such things as praying long prayers in public and deliberately displaying the physical effects of fasting, calling it 'hypocrisy', Matthew 5 and 6.

We are told to be honest one with another and to speak to anyone who has trespassed, privately, in order to avoid grudges, Matthew 18:15, not to pretend that nothing is wrong, if it is. There is no need to 'act' as if nothing was wrong, if indeed it is.