To do this, you need to set a boolean which will return true when you've talked to the character. Then create a conditional script for the line of dialog you want available after you've talked.
This script will set the boolean,
Code:
void main () {
SetLocalBoolean(HAS_TALKED,0,TRUE);
}
This script will check to see if you've talked yet,
Code:
int StartingConditional() {
int nTalked = GetLocalBoolean(HAS_TALKED,0);
return nTalked;
}
In the dialog file, put the dialog node you want the character to say AFTER you've talked before at the top. Then, in the conditional field, put the name of the second script.
In the node of dialog you want the character to say the first time you talk, you need to put the first script in the script field. You can put it on any line of dialog but you need to decide where it would be best suited.
Depending on the situation, you can make this as complicated or simple as you want, with many different conditional scripts.