It's arguable how canon it is but I really like how Harriman was fleshed out in some of the novels, there's that "Captain's Table" one where he has that conversation with that Romulan about the B's first mission and the death of Kirk, it's a nice rundown of how it affected him and how he's grown since then.
It's quite interesting to compare Kirk with Harriman, the new movies are quite a good way to see this contrast. Harriman's approach being to try and think around the problem, he certanily wasn't risk averse and was man enough to ask for help rather than let pride get people killed. Young Kirk would have covered up his insecurities and doubts with bravado and the outward appearance that he knew what he was doing, feeling that action, that being any action was better than doing nothing.
To be fair having a living legend watching your every move plus the media would make anyone intimidated!