Okay, I know I missed out on a lot of major characters, but I was trying to keep this short. I know a lot più of the actor's names, but here's what I put together for now...

HARRY POTTER CHARACTER & ACTOR:
Harry-Daniel Radcliffe
Hermione-Emma Watson
Ron-Rupert Grint
Snape-Alan Rickman
Fred and George- James and Oliver Phelps
Sirius- Gary Oldman
Umbridge- Imelda Staunton
Dumbledore-Richard Harris/Michael Gambon
Draco-Tom Felton
Ginny-Bonnie Wright
Bellatrix- Helena Bonham Carter
Voldemort-Ralph Fiennes
Lucius-Jason Isaacs
Neville-Matthew Lewis
Hagrid-Robbie Coltrane
Fleur- Clemence Poesy
Krum- Stanislav Lanevski
Cedric-Robert Pattinson
Luna- Evanna Lynch
Crabbe- Jamie Waylett
Goyle- Josh Herdman
Cho-Katie Leung
Filch-David Bradley
Mrs.Weasley-Julie Waters
Mr. Weasley- Mark Williams
McGonnagall- Maggie Smith
Flitwick- Warwick Davis
Lily- Geraldine Somerville
Seamus- Devon Murray
Dean- Alfred Enoch
James-Adrian Rawlins
Tonks- Natalia Tenna
Lavender- Jessie Cave
Padma- Afshan Azad
Parvati- Sitara Shah/Shefali Chowdhury
Lockhart- Kenneth Branagh
Slughorn- Jim Broadbent
Pettigrew- Timothy Spall
Moody- Brendan Gleeson
Quirrel- Ian Hart
Nearly Headless Nick- John Cleese
Bill- Domhnall Gleeson
Griphook- Warwick Davis
Vernon- Richard Griffiths
Petunia- Fiona Shaw
Dudley- Harry Melling
Colin- Hugh Mitchell
Barty Crouch Jr.- David Tennant
Trelawney- Emma Thompson
Susan- Eleanour Columbus
Scrimgeour- Bill Nighy
Scabior- Nick Moran
Grindelwald- Jamie Campbell Bower