What is the secret of comic acting and is it better to under play or overexpress for authentic 'laughs'?

Actor's have a saying, "Dying is Easy, Comedy is Hard". What that means is it for most actors drama is much easier to do than comedy.

Comedy requires spit second timing and truthfulness. You can't play the "comedy" you have to play the character as authentically invested in what is going around them. The laughs come from the character not seeing how foolish or silly their actions are.

Actors who "mug" their way through a show, and seeking laughs from the audience with facial tics aren't really very funny.

Timing, it's all in the timing.

Basically, the problem with comic acting is that, while there's a great variety of ways to make dialog in a drama work, there are only a few ways to get laughs from a scene in comedy. I think the secret is to keep rehearsing until you finally get to whatever works in the scene.

As the saying goes, comedy is serious business - a lot of work.

In response to, should you underplay or overact for authentic laughs:

That honestly depends on what style the comedy is. There isn't a "one technique fits all" approach to getting laughs. AJ is right, timing and truthfulness are usually the basic keys to it. If you're doing a small naturalistic style, obviously truthfulness is important but even in Commedia Del Arte and Pantomime, where you are over-expressing, it still needs to be rooted in truth.

Subverting expectations is also an important part of it. Often the choice of whether to overplay or underplay is based on what the audience will expect least.

It depends entirely on the play or show or movie.

The only secret is that the actor needs huge natural talent, years of first-class training and many, many years of experience.