With Karen's leaving of Doctor said to be "heartbreaking", Karen talks more about her leaving.
In a recent interview with IGN, she talked about why she thought it was the right time to leave:
"I wanted to go on a high. Also, Steven Moffat comes up with endless, amazing ideas anyway, but I wanted to make sure that I went on a high when the character was at her prime. There's just something quite nice and appealing about that to me. I don't know… It just felt right! I like to go on instinct."
She also said that she'd like it to be final;
"I really, really want to have a final exit and then be able to look back on that as a final exit. I don't know... I just don't want to take away from that exit by making cameos in the future. I'd quite like it to be final and for people just to remember it fondly."
Read the rest of the interview here