Run FCEUX. Engage TASEditor. Make sure readonly mode is on. Tap a key you've set for player 1. Note it updates TASEditor with the key you just tapped. If Recorder "All" option is selected in TASEditor, the script will use controls you set for all four players. If an individual player is selected for the Recorder, only player 1 controls are checked, but will toggle button presses of the selected player.
May seem odd, but the script requires readonly mode active in order to work properly. If, for some reason, you set the "Recording" check box, be sure to either stop the script or clear the "Auto function" check box. I have not defined a behavior for the script to follow when readonly mode is off.
It is very similar to
a script I posted a while back. The only real updates are that it reports what input changed in the history box, and that there are comments indicating where to stick your own code if you need something more. If, by some chance, the script picks up multiple pressed keys at the same time, it will report all pressed buttons in the history without problems.
Intended to reduce reliance on the mouse for precision edits of the input movie. I suppose you could TAS backwards with it if you really wanted to...