Jenna Coleman had originally planned to leave Doctor Who after season 8
Last year the rumors were swirling over the fate of Jenna Coleman’s Doctor Who character, Clara Oswald. It was highly believed that Coleman would be leaving the show at the end of the season, and no one said anything to quiet those rumors. It wasn’t until the annual Christmas special aired that we knew Coleman was sticking around for at least a little while longer.
Steven Moffat, in an interview with Doctor Who magazine, reveals that the rumors were true right up until the end — then Coleman had a change of heart. On the Christmas special, “Last Christmas,” Moffat states “that was her last episode, and then she asked me if she could be in [for] Christmas.” “For Christmas Day, it’s a bit nicer. Frankly, I didn’t want to lose her,” he said. “She’s an amazing actress and never stops working to make Clara better. I was very happy to go the extra mile to make sure we could keep her.”
(source)