BAD ACTOR

IN MY OPINION:

WILLIAM REYNOLDS

TRAITS OF A 'BAD ACTOR'

Pushing. Stanislavsky, the great Russian acting theorist, helped us understand that acting works better when actors pursue goals rather than try to emote. If that idea is new to you, and you'd like to learn more, see this post: Marcus Geduld's answer to What are some things that actors and actresses know that most people might not?
Bad actors think their job is to emote. And when they're "not feeling it," they force it. See Mel ("Give me back my son!") Gibson and Nicholas Cage.
And a:  Lack of confidence. It simply takes time to feel comfortable on stage or in front of a camera. There are many actors who will be wonderful some day, if they stick with the craft, but they simply don't have confidence yet. The stage, the camera, the moment, makes them nervous, which causes them to hold back. It cases them to protect themselves.  That means that you don't want to look vulnerable.  You want to look confident, and then the reverse happens, and the director, the audience sees right through it. 

Stick to what you are taught, not told by yet another actor trying to achieve the exact same thing.