"Just when everything seems to be going well between Sarah and Mark [Jason Ritter], Seth shows up and he's kind of in a darker place than he's been," creator Jason Katims told TVLine. "And so you see Sarah really having to struggle with wanting to help him and be there for him, but also at the same time [be fully present] in this relationship with Mark that's really good and healthy."
Also going on in the Braverman world: Brittany Belt will play Haddie's BFF who disapproves of her relationship with Alex, TVLine reports.
In other TV casting news ...
* 'Game of Thrones' has cast an actress to play Theon Greyjoy's sister Asha. Except that's not what her name will be. Gemma Whelan will play Yara, Theon's older sister. [EW]
* Syfy newbie 'Alphas' is beefing up its guest star roster.Appearing soon on the freshman series will be Summer Glau('Terminator: The Sarah Connor Chronicles'), Garret Dillahunt('Raising Hope'), Rebecca Mader('Lost') and more. [THR]
* NBC's 'Playboy Club' is giving Eddie Cibrian's Nick Dalton a sibling of sorts. Troy Garity will play John Bianchi, Nick's "brother" (in the crime family sense). [TVLine]
* Jason Priestley is returning to TV.The '90210' star will appear in four episodes of Syfy's 'Haven' -- and he'll direct an episode too. That Brandon Walsh, so multi-talented. [EW]
* ABC is saving 'GCB' for midseason, but the show is lining up its guest stars now. Tom Everett Scott has signed on for an arc as a nerd-turned-rich guy classmate of the main women. [Deadline]
* A little sibling rivalry is heading to MTV's 'Awkward.' 'Make It or Break It' star Zachary Abel will play Matty's brother on the new show. [TVLine]