2 solutions:
- The simplest, you don't add any case and you just add a "WindowSyncEx" line right after Kuja's line. With the "Ex" variant, you can choose the speaker so you don't need to have that line in a function of Zidane. There are a few things that can't be done with that solution thoug (I think there's no "RunAnimationEx" for instance).
- The cleanest, you add a case 142 indeed with your extra line and whatever happens at that time. You may add a case 142 to the other characters too (if people are reacting with an animation at the same time for instance). Then, you need to reroute the cutscene progression flow that is handled by a variable (surely "VAR_GlobInt16_47" in this case):
1) You want the new reaction to come right after Kuja's "case 49" script, so replace the "set VAR_GlobInt16_47 = 50" by "set VAR_GlobInt16_47 = 142",
2) In your new reaction case (142 but in only one character's script, Zidane's in this case), make it so the cutscene gets back on track after the added dialog by using "set VAR_GlobInt16_47 = 50".
That way, the cutscene's progression counter does not increase linearly but does something like "... 47, 48, 49, 142, 50, 51...". That's not optimal but that's less tedious than inserting a case and shift all the others. There *might* be side-effects because of that but I don't think there is (I've never seen a cutscene's script where it could be a problem).