I think it also helps tremendously when the manager also praises and supports said player. I've always liked Hughton as a manager, and he seems to be getting the real Garner out now, whereas at Watford, like you say, he looked nervous, and he never seemed to be an integral part of their team, at Forest he's certainly come on leaps and bounds from the first couple of matches.
He also seems to have been fully accepted, and trusted, by the Forest players as well, and that is a big plus.
The pre season will tell Ole if he is ready, but I think he may need at least another half season loan to be ready for the Prem, after all, in the position he seems destined to play in, experience certainly helps a lot.