Leia and Tarkin are definitely in the movie. Tarkin was a no doubter, but Leia has been confirmed by a release of the cast list.
The actor playing Tarkin is Guy Henry.
The actress playing Leia is Ingvild Deila.
It looks like both the actors are basically stand-ins overlayed with CGI to look like Cushing and Fisher.
Also, they got an actor to play General Dodonna. He's the guy who runs the base on Yavin IV. He gave the briefing on how to attack the Death Star in Star Wars: Episode IV - A New Hope.
Here is the list:
MUSIC BY -- Michael Giacchino
ORIGINAL STAR WARS MUSIC -- John Williams
JYN ERSO -- Felicity Jones
YOUNG JYN -- Beau Gadson
YOUNGER JYN -- Dolly Gadson
GALEN ERSO -- Mads Mikkelsen
LYRA ERSO -- Valene Kane
CASSIAN -- Diego Luna
K-2SO -- Alan Tudyk
CHIRRUT -- Donnie Yen
BAZE -- Jiang Wen
SAW -- Forest Whitaker
MON MOTHMA -- Genevieve O'Reilly
BAIL ORGANA -- Jimmy Smits
PRICESS LEIA -- Ingvild Deila... with Special Thanks to Carrie Fisher
DARTH VADER (VOICE) -- James Earl Jones
DARTH VADER -- Spencer Wilding & Daniel Naprous
KRENNIC -- Ben Mendelsohn
GOVERNOR TARKIN -- Guy Henry...with Special Thanks to Peter Cushing
GENERAL DODONNA -- Ian McElhinney (Ser Barristan!)
C3PO -- Anthony Daniels
2TUBES -- Aidan Cook
There were several more generals and senators listed, but no actors of note.
No R2-D2 listed in credits, but that could be because of Kenny Baker's death and there no longer being a little person inside.
It's either that or R2-D2 makes no appearance.
