There weren't 17 characters who appeared on both shows, but
18... see the list below:
- Andrew - A5.11, A5.20, Bs6, and Bs7... he also played Cyrus in B5.02
- Angel - As1-5, Bs1-3, B4.20, B5.07, B5.17, B7.21, and B7.22
- Anne - A2.12, A2.14, A5.22, B2.07, and B3.01
- Buffy - A1.08, A1.19, and Bs1-7
- Collins - A1.19, B4.15, and B4.16 (played by Alastair Duncan)
- Cordelia - As1-4, A5.12, and Bs1-3
- Darla - A1.15, A1.18, A1.22, As2, A3.01, A3.02, A3.03, A3.07, A3.08, A3.09, A4.17, A5.20, B1.01, B1.02, B1.07, B2.21, and B5.07
- Drusilla - A2.05, A2.07, A2.09, A2.10, A2.11, A5.08, A5.20, Bs2, B5.07, and B5.14 (B7.01, B7.10, B7.17... these were appearances as the First, so they don't count, but for completeness' sake, I included them.)
- Faith - A1.18, A1.19, A2.01, A4.13, A4.14, A4.15, Bs3, B7.18, B7.19, B7.20, B7.21, and B7.22
- Gypsy Man - A2.07 and B2.21 (played by Zitto Kazann)
- Harmony - A2.17, As5, B1.02, B1.11, B2.16, B3.09, B3.21, B3.22, B4.03, B4.07, B4.08, B4.22, B5.02, B5.04, B5.06, B5.07, and B5.14
- the Master - A2.07, Bs1, and B3.09 (B7.01 as the First)
- Oz - A1.03 and Bs2-4
- Smith - A1.19, B4.15, and B4.16 (played by Kevin Owens)
- Spike - A1.03, A2.07, As5, Bs2, B3.08, and Bs4-7
- Weatherby - A1.19, B4.15, and B4.16 (played by Kevin Owens)... he also played Spider Monster A4.20
- Wesley - As1-5 and Bs3
- Willow - A2.17, A2.22, A4.15, and Bs1-7
The list below is a list of people, who appeared on both shows, but played different characters... I've taken the liberty of underlining a few of the interesting ones.
- Micheal Beardsley - Accident Onlooker (uncredited) A1.06 - UC Sunnydale Student (uncredited) 4.05
- Andy Hallet - Lorne As2-5 - Student B4.10 (Uncredited)
- Carlos Jacott - Richard A1.07 - Ken B3.01
- Alex Skuby - Harlan A1.06 - Vincent B3.14
- Camden Toy - Prince of Lies A5.13 - Gentleman B4.10 - Gnarl-demon B7.03 - Turok-Han B7.09, 7.10, 7.11, and 7.15
- Andy Umberger - Dr. Ronald Meltzer A1.04 - D'Hoffryn B3.16, B4.09, B6.16, B7.05
- Joss Whedon - Numfar A2.21 - Newscaster (voice) B1.08
- Jonathan Woodward - Knox A4.22, A5.01, A5.05, A5.07, A5.14, A5.15, and A5.16 - Holden 'Webs' Webster B7.07
- John Patrick Clerkin - Black Robed Priest A1.15 - Man B6.09
- Bob Fimiani - Grounds Keeper A1.15 - Codger Demon 3.01 - Mr. Ward B4.21 and B4.22
- Mark Ginther - Head Demon Guy A1.15 - Lasovic A1.16 - Horned Demon 6.05
- Marc Rose - Mellish A1.16 - Josh B4.04
- Jennifer Slimko - Romanian Woman A1.18 - Vampire Girl #3 B5.01
- Derek Anthony - Dying Black Man A1.21 - Hotel Security Guard A2.05 - Imposing Demon B7.12
- Scott Berman - Vendor A1.21 - Mover #2 B5.01
- Todd Stashwick - Vocah A1.22 - M'Fashnik Demon B6.04
- Rob Boltin - Johnny Fontaine A2.01 - Soldier B4.22
- Edward James Gage - Mordar the Bentback A2.01 - Mover #1 B5.01
- Heath Castor - Shopper (uncredited) A2.04 - Cop #2 B2.19 - Party Guest (uncredited) B3.02 - Psych Student (uncredited) B4.01, B4.02, and B4.05
- David Meunier - Man A2.04 - Rat Faced Demon B6.05
- Tom Kiesche - Detective Broomsfield A2.08 - Vampire B5.22
- Norma Michaels - Aunt Helen A2.13 - Older Woman B3.16
- Darris Love - George A2.14 and A2.19 - Hallmate B4.04
- David Fury - First Worshipper A2.15 - Tropicana Guest (uncredited) A4.03 - Gregor Framkin A5.14 - Mustard Man B6.07 and B7.05
- Roger Stoneburner - Vampire A2.17 - Vampire B2.13 - Vampire B2.22
- Adam Weiner - Caged Guy A2.17 - Simon B6.09
- Persia White - Agnes 'Aggie' Belfleur A2.20 - Aura B1.01
- Whitney Dylan - Serving Slave A2.22 - Lysette Torchio 3.13
- Bob Morrisey - Dr. Gregson A3.01 - Homeless Man/Crazy #1 B5.02, B5.20, and B5.21
- Kal Penn - Fez Boy A3.02 - Hunt B4.05
- Justin Shilton - Billy Blim A3.02 and A3.06 - Monroe B7.19
- Stephen W. Bailey - Ryan A3.04 - Cave Demon B6.20, B6.21, and B6.22
- Jack Conley - Sahjhan A3.07, A3.08, A3.09, A3.10, A3.15, A3.16, A3.17, and A5.18 - Cain B2.15
- Christopher Leps - the Assassin A3.08 - Vampire A5.01 - Agent Goodman B5.04
- Marti Noxon - Cordy' theme tune singer (voice) (uncredited) A3.11 - Parking Ticket Woman B6.07 and B7.05
- Alan Henry Brown - Lead Nandrah A3.12 - Funeral Director B5.17 - Demon Bartender B6.20
- Brett Wagner - Nandrah Prince A3.12 - Bong'dar Demon A4.13 - Trucker B6.22
- Bernard K. Addison - Root Monster A3.14 - Cop #1 B6.13
- Annie Talbot - Mother #2 A3.15 - Lady With Baby B5.17 (can anyone confirm these are in fact two different charaters?)
- Jeff Denton - Lead Guitar A3.16 - Vampire B7.01
- Bryan Friday - Holtzian A3.16 - Demon #1 B6.03
- P.B. Hutton - Mrs. Ruzaove A3.18 - Mrs. Kalish B2.09
- Noel Gugliemi - Driver Vamp A4.01 - Vince B6.05
- troy Brenna - Plated Demon A4.02 - Nezzla Demon B6.19 - Avilas Demon B7.04
- John Collela - Croupier A4.03 - Police Officer B7.08
- Spice Williams - Debbie A4.13 - Patrice B2.10
- Thom Williams - Possesed Human A4.19 - Vampire B7.17
- Erik Betts - the Keeper A4.21 - Agent Brown B5.04 - Assasin #2 B5.12 - Predator Demon B.05 - Vamp #4 B6.08 - Vampire B7.08 - Demon #2 B7.15 - Vamp #2 B7.16 - Uber-vamp B7.21 and B7.22
- Bonita Friedericy - Patience A4.21 - Manager/Mrs. Finkle B3.19 and B3.20 (I think these are the same characters... any help?)
- Leland Crooke - Archduke Sebassis A5.05, A5.21, and A5.22 - Prof. Lillian B5.19
- Bruno Gioiello - Security Guard A5.06 - Captured Bringer B7.20
- Abigail Mavity - Hannah A5.14 - Sara Harris, Age 8 B6.16
I think the list is complete now... it took me a whole lot of work, so I hope you appreciate it
😀
