It's pretty powerful to look someone right in the eyes. Imagine having to do it for an indefinite amount of time. To sit, in quiet, is a difficult task on it's own. To look at someone while you're doing it...well, it adds a challenge.
We don't often think about looking people in the eye: The guy that isn't your waiter, but brings the water to your table, the person that holds a door open for you as you're entering the bank, the neighbor who's walking their dog, the cashier of anywhere you go, and that little kid that's staring at you in wonder as she gets dragged past you in the crosswalk. As a collective, we tend to avoid eye contact, opting for the easier look down, by, past or over someone. That head on connection can feel overly-intimate for some people,
the recent show at MOMA. People that sit and stare in the artist's face for whatever amount of time they're comfortable, or uncomfortable with, and appear to be quite moved (extremely, in some cases), with absolutely no words involved.
Watching Dennis Hopper stare into the lens, saying nothing, changing expression, looking away. You could practically write the script for that inner dialogue.