You can probably use message_current variable to check what line it is in. For example:
if (textbox.message_current == 2) {
// Do stuff
}
Let me know if that helps. I also have a Cutscene engine that could make making cutscenes easier.
Edit: Also, I think it would be easier to first do some dialogue, then some other cutscene stuff and then the rest of the dialogue.
Sidenote: I edited your post to use code block formatting.