I will be keeping the same cast as in the movie but here are the ones that will be present. There are the covens, the main characters as well as those that Roxanne knew when she was a human.

Coven Volturi.

Alec Volturi > Cameron Bright

Caïus Volturi > Jamie Campbell Bower

Aro Volturi > Michael Sheen

Marcus Volturi > Christopher Heyerdahl

Demetri Volturi > Charlie Bewley

Jane Volturi > Dakota Fanning

Felix Volturi > Daniel Cudmore

Heidi Volturi > Noot Seear

There will be appearances from Chelsea, Corin, Afton, Sulpicia, Athenodora, Santiago and more among the Volturi!

Roxanne Williams - Volturi > Miranda Kerr

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Coven Cullen.

Carlisle Cullen > Peter Facinelli

Esme Cullen > Elizabeth Reaser

Emmett Cullen > Kellan Lutz

Rosalie Cullen > Nikki Reed

Alice Cullen > Ashley Greene

Jasper Cullen > Jackson Rathbone

Isabella Cullen > Kristen Stewart

Edward Cullen > Robert Pattinson 

Renesmee Cullen > Mackenzie Foy

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Werewolf.

Jacob Black > Taylor Lautner

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People who have been part of Roxanne's human life

There may be surprise appearances among the cast of Twilight and also characters who do not have faces at the moment: