Twenty four years ago I was the 200 wonder training on the A320. As I got into the sim for the first time the Trainer patted me on the shoulder and said "I love the look on the faces of all you youngsters the first time in here. The self doubt and awe is so obvious." He then proceded to take me and a load of others from being a 200 hour trainee through to being an airline pilot by showing us how to do it, leading us gently on and occasionally yelling "**** Me Boy!! if you keep that up you'll never make it! Now do it again and work harder!"
The line trainers showed us how to operate the aircraft and gave us a lot of tricks of the trade and the rest was willingly passed on by the line captains we flew with.
All these years on I'm pretty convinced that all the good qualities you look for in a trainer are also vital in the line captains too. After all once base and line training are over the learning how to make duty hours work on a day where everything slips is vital, local knowledge about airports makes life easier and and in depth knowledge of company procedures can really only be learned on the job.