Why give Effie Trinket such a major role in Mockingjay?

Question

In the Mockingjay book, Effie Trinket has only a tiny bit part at the end, where she appears as a rather unconvincing 'rebel' to help Katniss with her costume and preparation for the

assassination of the president (but which one?)

In the Mockingjay films, she has a much more major role, replacing the book's "Fulvia" as Katniss's District 13 stylist throughout.

Has any official reason ever been given for this change?

An obvious reason is that the filmmakers didn't want to underuse an actress of Elizabeth Banks's profile and talent (although they've done exactly that with Gwendoline Christie in Mockingjay Part 2), but have they ever said explicitly that this is the reason? There are various other possibilities, such as that they didn't want to introduce yet another new face in District 13, or for the rapport between Effie and Haymitch

(ahem).

Answer 1

The writer and director liked Effie and wanted to keep her in the story.

According to the actress, Elizabeth Banks:

"Well she [Effie] doesn't make an appearance in District 13 in the actual book, but Suzanne Collins and Francis [Lawrence] and I…everyone sort of came to me and said ‘We really feel like Effie…it would be really fun to keep her involved in the story' and of course I'm thrilled to be a part of it and get to go to 13 and take care of Katniss a little bit longer."

Answer 2

For at least one scene, she apparently took the place of what was originally planned to be Plutarch due to the death of Philip Seymour Hoffman. From Nina Jacobson's answer to this Quora question (How did the tragic death of Philip Seymour Hoffman alter the Hunger Games franchise?):

Practically, Phil had shot about 80% of his scenes. What we had to do with the remaining 20% was to give two key scenes to Liz Banks and Woody Harrelson. In MJ1, Effie gives Katniss Cinna's design for the mockingly uniform instead of Plutarch, as was originally scripted. In MJ2, Haymitch reads a letter in a scene that, in the book and the original script, takes places between Katniss and Plutarch. Without a doubt, Phil would be a bigger presence in the movie if he had stayed with us.

I came across this quotation by reading Roger's answer to this question from Movies SE: What changes to Mockingjay Part 2 were made due to P. S. Hoffman's passing?

This is also mentioned in the Youtube CineFix video 7 MORE Things You (Probably) Didn’t Know About The Hunger Games! that calccrypto linked to in the comments.