ON ACTING: Beware the Constant Smile
I argue that this compulsive habit of 'smiling' under conflict/duress, the obsessive mannerism of smiling through the majority of all moments of any acted scenes, is not bravery, but the very opposite: it is a refusal by the actor to get involved with the scene. It is an unconscious way of saying (and behaving): "I can handle this. This isn't that filled with import. Watch me smile at what your saying and doing." The 'smiley' reaction is the coward's mask of retreat. It sends a signal the other character--and to the audience--and, most counter-productively, to the actor himself/herself--that the scene is not important enough to the actor to feel.
In life...think of when we smile at someone with whom we are involved in a conflictual situation, we do so as a tactical ploy...to convince an opponent that they are not winning; conversely, we are so sure of victory, of the propriety of our position (along with the stupidity and light fragile nature of theirs) that we are forced to smile, to dismiss them and their position/argument. We issue a benign, superior upturn of the lips and a toothy grin.. Not a bad tactic, that; it often works on insecure opponents.
But it is is an unproductive habit/tactic/choice for an actor to overly embrace in a scene, because it often leads to emotionally distancing the 'smiley' actor from the scene (which is probably why the actor makes that behavioral choice in the first place! They don't want to take the scene too seriously because he/she doesn't want to feel!) When the action of 'smiling' in a scene becomes a compulsive reactive manner (which is a good definition of the pejorative term 'mannerism', by the way), I would advise the actor to toss the 'smiley mask' away for a rehearsal or two; to toss the upturned lips aside, far out of their reactive arsenal. In doing so they may be forced to confront a scene's conflictual challenge head on, seriously, and be forced to use, experience and display other emotional weapons in their human system's arsenal...and be more varied and interesting in their tactics--and performance--as a result!