Actor pushes emotion beyond credible range — overexaggerated gestures, voice projection, facial expressions. Medium to close-up kills it; long shot might carry it.
On set, you recognize over-acting immediately: The actor squints as if staring into the light, throws their arms up like a bird, and speaks every word with theatrical elongation. The camera—especially in a close-up—doesn't forgive this. It sees every strain, every muscle cramp, every artificial movement. What might still be convincing on stage 30 meters away becomes a parody in front of a 35mm lens.
The problem usually arises from insecurity or incorrect direction. An actor who doesn't know how subtle film acting needs to be compensates with volume and gestures. Your job as a director is then to curb this—not through criticism, but through trust. "Less. That was too much. Try again, but as if no one were watching you." The best rule: What you see in the monitor in a close-up is already too much. Resist it.
Over-acting also occurs due to the wrong distance between the camera and the performer. An actor who played for a long shot will appear too large if you suddenly cut to a close-up. The proportions no longer fit. The solution here: Clarify in pre-production which shot sizes will be filmed and calibrate the actor accordingly. In a wide shot, you can play bigger; the closer the camera, the more subdued the performance. This isn't a rule, but physics of the medium.
Sometimes, over-acting is consciously needed—in comedies, horror, or when the staging is absurdist. Then it's not over-acting, but style. The difference lies in intent. If you want it, you call it "stylized"; if it happens because the actor doesn't know what they're doing, it's a mistake. In the edit, you'll then notice if the scene works or if you need to shoot re-takes.
Tip: Let actors play multiple takes—one internal, one normal, one over-acted. In the edit, you'll then have a choice. Sometimes the version that seemed too quiet on set works perfectly in the edited context.