Here's a list of actors from the Downton Abbey film (2019) that I have seen on stage (Mostly London theatres)
This list is unfinished and may be modified from time to time
If you have your own list let me know and I will post a link here
01 - Lady Mary Talbot - Michelle Dockery - The UN Inspector — Olivier Theatre (2005)
02 - Lucy Smith - Tuppence Middleton - The One — Soho Theatre (2018)
03 - Violet Crawley, Dowager Countess of Grantham - Maggie Smith - Three Tall Women — Wyndhams Theatre (1995)
04 - Cora Crawley, Countess of Grantham - Elizabeth McGovern - The Misanthrope — Young Vic (1996)
05 - Edith Pelham, Marchioness of Hexham - Laura Carmichael - Uncle Vanya — Vaudeville Theatre (2012)
06 - Tom Branson - Allen Leech - Ecstasy — Hampstead Theatre (2011)
07 - Anna Bates - Joanne Froggatt - The Knowledge — Bush Theatre (2011)
08 - Lady Bagshaw - Imelda Staunton - Bold Girls — Hampstead Theatre (1991)
09 - Phyllis Baxter - Raquel Cassidy - Sunday Father — Hampstead Theatre (2003)
10 - Mrs. Hughes - Phyllis Logan - Gaucho — Hampstead Theatre (1994)
11 - Daisy Mason - Sophie McShera - The Entertainer — Garrick Theatre (2016)
12 - John Bates - Brendan Coyle - The Weir — Royal Court (1998)
13 - Robert Crawley, Earl of Grantham - Hugh Bonneville - US and Them — Hampstead Theatre (2003)
14 - Queen Mary - Geraldine James - Give Me Your Answer - Do! — Hampstead Theatre (1998)
15 - Charles Carson - Jim Carter - Fashion — Tricycle Theatre (1990)
16 - King George V - Simon Jones - The Hitchhikers Guide to The Galaxy Radios Show... Live! — Churchill Theatre, Bromley (2012)
17 - Isobel Grey, Baroness Merton - Penelope Wilton - The Cherry Orchard — Albery Theatre (1996)
18 - Captain Chetwode - Stephen Campbell Moore - Coriolanus — Gainsborough Studios (2000)
19 - Mrs. Patmore - Lesley Nicol - The York Realist — Donmar Warehouse (2018)
20 - Mr. Wilson - David Haig - The Recruiting Officer — Royal Court (1988)
21 - Andy Parker - Michael Fox - A Lie of the Mind — Southwark Playhouse (2017)
22 - Bertie Pelham - Harry Hadden-Paton - Posh — Royal Court Theatre (2010)
23 - Joseph Molesley - Kevin Doyle - A Midsummer Night's Dream — Barbican Theatre (1995)
24 - Monsieur Courbet - Philippe Spall - Against — Almeida Theatre (2017)
25 - Nanny Harewood - Alice McCarthy - Rotterdam — Trafalgar Studios (2016)
26 - Mrs. Webb - Richenda Carey - The Roundabout — Park Theatre (2016)
27 - Henry Lascelles - Andrew Havill - A Midsummer Night's Dream — Barbican Theatre (1989)
28 - Detective - Darren Strange - The Tempest — Barbican Theatre (1998)