When do certain scripts run?

[Ian Leigh]

I am putting together a simple program which scans a directory and  
lists the files contained therein in a scrolling list field. I am  
wondering when the script of that field would actually get called?  
For example, I would like to put a filter on it but I can't do it by  
putting the filter command in the script for the field, which seems a  
bit illogical. At that point are scripts for a scroll list actually run?

Cheers

Ian