Celebrated film editor Walter Murch famously said that you should almost always cut on the actor’s blink; and less famously explained that you never cut on the actual blink, but sometimes before and only when it made sense for other reasons. What I’ve never heard explained is why that technique so often works: it’s because unlike people in daily life, people on camera usually blink intentionally.
For actors, dialogue and physical movement are both considered actions. Script analysis reveals each character's objective, and the various actions they take to obtain their goal. They speak in order to reach their goal, to convince another character do something. Actors will identify the beats (or sub actions) that determined when an action is complete. The action, for example, may be to intimidate the other person. They'll accomplish this by probing their vulnerabilities, insinuating that harm might come to them, and escalating to a blunt threat. Each sub-action (probe, insinuate, threaten) is a beat that makes up the action, and they'll only give up on a sub-action when it's clear their not reaching their objective. They’ll push through each beat with 100% intensity until its done. Then they’ll blink. It's a divider, a rest, before they begin their next action. The actor’s “beat,” therefore often coincide with cuts. What this means is that you can often predict when an actor will blink just by analyzing the script for beats. Though you'd never want to edit so blindly, you could almost edit without watching the actor at all.
Actors are trained to do analyze beats and actions (Stanislavsky is the most famous teacher of this technique), but non-actors also do a gentler version of action-blink when they know a camera is pointed their direction. People tend to become more directed on camera. No one wants to ramble or fumble or stare meaninglessly into space. So they make sure there’s a reason for everything they say. An action.
A blink is a “rest” on several levels: resting the eyes, arresting the action. As editors, though, we often want to keep forward motion of the action, which is another reason why cutting just before the blink works. It keeps the action moving at top pace. What’s fascinating (and rarely seen in movies) is when an actor blinks as an action, rather than a rest. Takeshi Kitano in Merry Christmas Mr. Lawrence is a wonderful example of that, in a film that's getting a second life of sorts. Next time you watch a movie, look at when the actor blinks, what it means, and why the editor chose to cut or not cut.