All bad acting stems from a similar source: the bad actors' lack of confidence or refusal to stand in their own light. By that I mean, such actors refuse to fully be themselves (the aspects of themselves called for by the script) and perform with freedom and fullness. Such actor timidity in the face of performance demands results in filling the performance void by false acting; which has many guises, masks and forms: for example not really listening to the other actors in the scene, or a refusal to be really, emotionally and personally affected by the events encountered in the scene (including the other character's dialogue), or a constant consciousness of one's own performance rather than a focus on the events of the scene, most especially the other characters/actors in the scene. Other bad acting habits include the actors' auto-stimulating of emotions rather than, as in real life and in good performing, having emotions activated by the external events of the scene; or emotionally and reactively anticipating the events in a scene before the events actually occur; and/or highlighting one's actions in a scene for the audience consumption rather than in conformance to the inner logic in the playing of the scene.
As I say, all bad acting habits result from a lack of confidence, often heightened by a lack of understanding of, or appreciation of, the power of personal emotional truth and performance reality; sometimes exacerbated by bad instruction (teaching) or direction. A good actor therefore needs practice, experience, and proper instruction; and all that eventually melds into a free flowing and powerful performance ease. An auditioner can tell a good actor when they walk into the room, The good actor exhibits confidence in their self and craft and it manifests itself in what they call "presence"; presence simply meaning the actor's willingness to confidently state (without words): "Hi. I'm present. What do you need?"