Ben Barnes as
Sirius Orion Black / Padfoot/ Snuffles
Charlie Heaton as
Peter Pettigrew/Wormtail
Andrew Garfield as
Remus John Lupin/Moony/Rem
----------
and of course
Timothee Chalamet as
Regulus Arcturus Black/ Reggie/ Reg
James Phelps & Oliver Phelps as
Frederic Gideon Weasley/ Fred/Freddie/ Gred
&
George Fabien Weasley/ Georgie/ Forge
-and everyone as their respective casting/fancast later on the years