I tend to assume that the cast of recent and upcoming films are contractually obligated to put a few extra lines on file for various games and tie-ins... and since the majority of featured characters are either from current, recent or sequel properties, I imagine that it was 'easier' still to get quite so many back.
Which is not to suggest it was simple, and such speculation goes hand-in-hand with a great deal of praise and thanks to all involved! Good job.
(Next question: is every line heard original to the game, or are there samples and clips used from the various movies
as well..?)