I tried a Popover, which was great, except it makes a one-to-one relationship with the films. If I could script that to open a New Record linked current film, that would be perfect.
Not sure what you mean by that; opening a popover by itself doesn't do anything, except … well, open a popover and just leave you (still) in the Movies context.
There several different ways to do what you want to achieve, but while you're at a popover – why not use one with a global field and two button (Add, Cancel)?
This allows you to do a bit of post-processing before actually creating the new review. If you're satisfied, capture the movieID, close the popover, create a new related review, set the text field to the global, clear the global and go back to your movie layout (or do whatever action your workflow requires next).
If you're concerned about the user existing the popover prematurely, there are several techniques using a global $$var to prevent this (as described by a number of posts in this forum).
If you want the opening of a new window to force the user to execute only whatever process you wish them to address in the contxt of that new window, why not make the new window modal? That way nothing outside that modal window can be addressed until the window is dismissed.
Objective: From a Layout of Films, I want a user to press a button to add a new review.
The tables are Films and Reviews linked by Film ID. There are many reviews to one film, but I don't want the user to see any other reviews. They should only see a New Record Layout.
I've tried a New Window, and it's good for opening a New Record in the Review Layout, but I can figure out how to Disable Navigation, so the user can only see the New Record.
You can disable record navigation by using the Custom Menus feature of FileMaker Pro Advanced.
On the menu set used by the New Review Layout, remove the Records menu. This will disable the ability to create new records, delete records, or navigate to any other record. Removing the menu (or just the corresponding submenu items) will also disable the on-screen buttons and will disable the key shortcuts too.