Weez, revstan and trpw and try to make it an official one online. Like them I'll use this as my definitive cast list, it seems to include people who ended up on the cutting room floor, so I guess Toby Regbo counts.
I'll probably come back to this over time as I add more or realise there's someone I'd missed. If I've seen someone on stage loads of times I may well forget the odd production I've seen, but after all it only takes one show for it to count. Unfortunately this isn't a list that's physically possible to complete, what with Richard Harris not waiting for me to see him on stage before dying.
I've linked to my reviews of the shows where possible; any title that isn't a link means it's from before I started reviewing online.
1 - Daniel Radcliffe (Harry Potter) - Equus, How to Succeed in Business Without Really Trying.
2 - Carolyn Pickles (Charity Burbage) - Fram.
3 -Alfred Burke (Armando Dippet) - Oedipus.
4 - Michael Gambon (Albus Dumbledore) - Volpone, Henry IV Parts 1 & 2, No Man's Land, Krapp's Last Tape, All That Fall.
5 - Toby Regbo (Young Albus Dumbledore) - Tusk Tusk.
6 - David Bradley (Argus Filch) - Henry IV Parts 1 & 2, No Man's Land, Moonlight.
7 - Zoë Wanamaker (Rolanda Hooch) - The Glass Menagerie, Much Ado About Nothing, All My Sons, The Cherry Orchard.
8 - Kenneth Branagh (Gilderoy Lockhart) - Ivanov.
9 - Maggie Smith (Minerva McGonagall) - The Lady In The Van.
10 - Gemma Jones (Poppy Pomfrey) - Richard III, The Turn of the Screw.
11 - Ian Hart - (Quirinus Quirrell) - Three More Sleepless Nights, Speaking In Tongues.
12 - Jim Broadbent (Horace Slughorn) - Theatre of Blood.
13 - Miriam Margolyes (Pomona Sprout) - Wicked, Endgame.
14 - Jessie Cave (Lavender Brown) - Arcadia, Mary Rose.
15 - Katie Leung (Cho Chang) - Wild Swans.
16 - Henry Lloyd-Hughes (Roger Davies) - Punk Rock, Rope, Posh, Tiger Country, The Changeling, NSFW.
17 - Hugh Mitchell (Colin Creevey) - Arcadia.
18 - Isabella Laughland (Leanne) - Greenland, Wanderlust, The Last of the Haussmans.
19 - Matthew Lewis (Neville Longbottom) - Our Boys.
20 - Alfred Enoch (Dean Thomas) - Happy New, Antigone, Timon of Athens.
21 - David Brett (Dedalus Diggle) - The Four Stages of Cruelty, Outward Bound, The Cabinet of Dr. Caligari.
22 - Ciarán Hinds (Aberforth Dumbledore) - Burnt by the Sun, Juno and the Paycock.
23 - Kathryn Hunter (Arabella Figg) - Fragments, Kafka's Monkey, Antony and Cleopatra, Tell Them That I Am Young and Beautiful, The Bee, A Tender Thing, Mare Rider.
24 - Robbie Jarvis (Young James Potter) - Shivered.
25 - Geraldine Somerville (Lily Potter) - Serenading Louie.
26 - George Harris (Kingsley Shacklebolt) - Frankenstein.
27 - Mark Williams (Arthur Weasley) - As You Like It.
28 - Domhnall Gleeson (Bill Weasley) - Now or Later.
29 - Julie Walters (Molly Weasley) - The Last of the Haussmans.
30 - Ralph Fiennes (Lord Voldemort) - God of Carnage, Oedipus, The Tempest.
31 - Suzie Toase (Alecto Carrow) - Talent, One Man Two Guvnors.
32 - David Tennant (Barty Crouch Jr) - What the Butler Saw, Much Ado About Nothing.
33 - Jason Isaacs (Lucius Malfoy) - The Dumb Waiter.
34 - Helen McCrory (Narcissa Malfoy) - The Seagull, Rosmersholm, The Late Middle Classes, The Last of the Haussmans.
35 - Sian Thomas (Amelia Bones) - Fram, Spring Awakening, Britannicus, Blue Heart Afternoon.
36 - Kate Fleetwood (Mary Cattermole) - Champagne and Kippers*, Medea, Macbeth, Life Is A Dream, London Road, Our New Girl.
37 - Steffan Rhodri - (Reg Cattermole) - Clybourne Park, The Kitchen Sink, Posh.
38 - Roger Lloyd-Pack (Barty Crouch Sr) - When We Are Married, Dealer's Choice, Twelfth Night, Richard III.
39 - Jeff Rawle (Amos Diggory) - Fram, The Power of Yes, Hymn and Cocktail Sticks.
40 - Jessica Hynes (Mafalda Hopkirk - voice) - The Norman Conquests.
41 - Sophie Thompson (Mafalda Hopkirk) - As You Like It, Clybourne Park, She Stoops to Conquer, The Physicists.
42 - Bill Nighy (Rufus Scrimgeour) - The Seagull.
43 - Guy Henry (Pius Thicknesse) - King John, Twelfth Night.
44 - Imelda Staunton (Dolores Umbridge) - Entertaining Mr Sloane, Sweeney Todd.
45 - Harry Melling (Dudley Dursley) - Mother Courage and her Children, Women Beware Women, The School for Scandal, When Did You Last See My Mother?, The Starry Messenger, I Am a Camera.
46 - Fiona Shaw (Petunia Dursley) - Medea, Mother Courage and her Children, London Assurance, Scenes From An Execution, The Rime of the Ancient Mariner.
47 - Richard Griffiths (Vernon Dursley) - The History Boys, Equus, The Habit of Art, The Sunshine Boys.
48 - Ian Kelly (Mr Granger) - The Pitmen Painters.
49 - Michelle Fairley (Mrs Granger) - Greta Garbo Came to Donegal.
50 - Jim Norton (Mr Mason) - The Veil.
51 - Frances de la Tour (Olympe Maxime) - The History Boys, The Habit of Art, People.
52 - Dawn French (The Fat Lady) - When We Are Married.
53 - Kelly Macdonald (The Grey Lady) - Hurlyburly.
54 - John Atterbury (Phineas Nigellus Black) - Ivanov.
55 - Rhys Ifans (Xenophilius Lovegood) - Under Milk Wood, Volpone.
56 - Lee Ingleby (Stan Shunpike) - Our Class.
57 - Miranda Richardson (Rita Skeeter) - Grasses of a Thousand Colours.
58 - Paul Ritter (Eldred Worple) - The Norman Conquests, The Curious Incident of the Dog in the Night-Time, The Audience.
59 - Toby Jones (Dobby - voice) - The Play What I Wrote, Parlour Song, The Painter.
60 - Ray Fearon (Firenze - voice) - Julius Caesar.
61 - Simon McBurney (Kreacher - voice) - Measure for Measure, Endgame.
62 - Michael Wildman (Magorian) - Hamlet, Welcome to Thebes.
63 - Lenny Henry (Shrunken Head - voice) - The Comedy of Errors.
*The one instance where I wasn't so much "spotting" a future Potter actor as co-starring with one. Basically, I win the internet.