To answer the original question I would reply, as many have done so already, that patience is probably the most important quality for a trainer to have. That and a genuine desire to teach another human being a new skill.
As to the "man and boy "concept where the trainer is a God-like entity who must not be questioned it would be all well and good except it doesn't/didn't work. NSF highlighted the Staines crash as a good example where it is believed that an aggressive over bearing Capt might have made an under confident FO unlikely to speak up when he needed to.
It's quite possible that a brand new FO, even during line training, will spot something wrong and should have the confidence to speak up. A good trainer will use that as an example of why it's a 2(3) crew aircraft and reinforce the message that one should never keep silent if something appears amiss.