How do we do that? Say I have a video of someones face and then at the end I want to freeze it for a few seconds.

You don't need to capture a still image. There's several ways of doing this. However, the approach I generally take is to add a Velocity Envelope to the clip and then change the speed to 0% at that frame. You'll need to add two points on the envelope - one at the original speed and one at 0% right next to each other. If you don't want it to suddenly stop, you can spread the points farther apart and it will gradually slow down.

I also have an article in one of my newsletters (http://www.jetdv.com) that talks about changing the speed of a clip. You might find all of them helpful as well.

What frame rate needs to be used when filming?
The problem I've been trying to solve is 60i footage.
When I get back to the studio I'll try that envelope technique.
Trying to capture clean stills from 60i is a bear.
I filmed some 30F, and it's a little better, but can't
use the footage for slow motion.
Too jittery.