Nora Zehetner (pictured below) and Robert Baker have been cast as two of the recurring residents surfacing in Episode 5 of the upcoming season of Grey's Anatomy.

Zehetner, familiar to TV fans from her runs on Heroes and Everwood, will play a Seattle Grace doctor described by a source as "feisty, very smart and competitive."

She is poised to butt heads with at least one member of Meredith Grey's posse. Which one(s) of the original interns-turned-residents she feuds with is unclear.

Meanwhile, Robert Baker, from The CW's short-lived Valentine, has been cast as the seemingly nicest of the new residents, who may be hiding a nastier side.

The casting of the docs comes as Grey's Anatomy prepares to bid adieu to George O'Malley (in the season premiere) and accomodate Ellen Pompeo's maternity leave.

Krista Vernoff, co-executive producer of the program, recently stated that Mer-Der is still the focus of the show, but Ellen will nevertheless miss some time this year.

Precisely why the new residents' arrive is the big mystery here, and one that Shonda Rhimes isn't willing to reveal. At least until her next Twitter or blog posting.

Scrubbing in alongside Zehetner and Baker will be Jesse Williams (Beyond the Break, the Joss Whedon thriller The Cabin in the Woods) as a charismatic caregiver.

Steve Marsi is the Managing Editor of TV Fanatic. Follow him on Google+ or email him here.