According to her Facebook page, Karen David will have a recurring guest role on Castle.
Karen David: Okay so here's my news...I started filming yesterday and for the next month, I'll be filming on the ABC series, "Castle"! How many of you watch the show? I'm really excited to be playing through a great storyline! xo
Thanks Guys and Gals. I'm really excited about my storyline and will be in a few episodes of the show for now. I play Columbia University, sophomore student, Sara. I can't say too much, but let's just say she gets caught up in something big in the storyline! lol #Castle
Source: Karen David's Facebook page
[I have a feeling this has something to do with the "big storyline" involving Martha and Alexis that Tamala mentioned in her interviews, maybe even the two-parter.]