Did you know that the FileMaker training series uses Actors and Movies to illustrate how to work with many to many relationships?
That's what you have here.
You need a third table so that any given actor can be linked to more than one movie and and any given movie can be linked to more than one actor.
Movies----<Role>----Actors (---< stands for "one to many")
Movies::MovieID = Role::MovieID
Actors::ActorID = Role::ActorID
With this setup, a portal to Role on the Movies layout can be used to assign existing actor records to that movie. A portal to Role on the Actors layout can be used to assign existing Movie records to that actor. Fields defined in the Role table can document the actor's role in that movie (such as the character's name.)
Drop downs or pop ups in the portal would be used to select the actor on the Movie layout and the Movie when on the actor layout. The value lists for these fields would list the ID number as field 1 and the name as field 2. The actor's name field from Actors can be added to this portal on the Movie layout and the Movie Title from Movies can be added to the portal on the Actors layout so that the "name" of the selected actor/movie is listed in the portal.