Alex O’Loughlin played Mick St. John in the short lived Moonlight before Steve McGarrett, Daniel Dae Kim played Jin Kwon in Lost before Chin Ho Kelly, Grace Park played as Lt. Sharon ‘Boomer’ Valerii in Battlestar Galactica before Kono Kalakaua.
Well this one was almost too easy but before David Duchovny was Hank Moody he was Agent Fox Mulder on The X-Files.
Before James Murray was Billy Collins he was Stephen Hart in Primeval.
Before David Boreanez played FBI Agent Seeley Booth he was a VERY broody vamp in Buffy and Angel.
Before Vincent Kartheiser was Pete Campbell he was Connor on Angel, and just for fun lets not forget Christina Hendricks who was the unforgettable Saffron on Firefly even if she was only in 2 episodes.
How I Met Your Mother
Before Alyson Hannigan played Lily Aldrin she was a powerful witch on Buffy. Also this show has used many other Whedon favorites from Alexis Denisof (Buffy / Angel), Harry Groener (Buffy) and Amy Acker (Angel) as one-off or reoccurring characters.
Before Nathan Fillion was Rick Castle he was Captain Malcolm ‘Mal’ Reynolds aka “Captain Tight Pants” in Firefly.
Before Gina Torres played Jessica Pearson she was the ass kicking Zoë Washburne on Firefly.
Before Adam Baldwin played John Casey he was the “Hero of Canton” Jayne Cobb on Firefly.
Best for last, holy crap is that the pink ranger with a real gun? Yup WAY before Amy Jo Johnson was Jules Callaghan she was Kimberly Hart of the Pink Ranger in the original Mighty Morphin Power Rangers.
* Just a note There is a lot of Whedonverse actors on here and many have made reoccurring appearances in his other shows, I have not made all of those noted. Also there were a few actors I left off of the list because they were supporting characters, I tried to go with leads. However there may be some I missed please comment below and let me know.