Not sure I’m following you. You can use the space bar to stop.
Are you using spectramatte to key?
Color drop the green screen (you are on the top video layer, right?) then tweek the peramters as needed. If you advance it to 3D, you’ll have a little more control over edges, ect.
If spectramatte cant handle it, keylight in AE is wonderful. You can use a simple color key to get rid of most of the bknd then use keylight for the rest (less processing).
