According to Steven Moffat, the search for the next Doctor began and ended with Capaldi. He downloaded a couple scripts, put on a brief persona that fit each script, and recorded his test. That was it, apparently. He was The Doctor. There was no doubt about it. Moffat also revealed that Capaldi was in the running for the Eleventh Doctor, but it was given to Matt Smith instead. Moffat felt that the timing wasn't right for Capaldi back then, but he's had him in his mind throughout Smith's reign. So when it came time to search for twelve, he went back to Capaldi and the rest will be history soon enough.