You don't pretend, is how it works. You act as someone would act until they become you. Then it's not acting. Think of the trope "becoming the mask" as a starting point. But it needs more than just thinking "I'm going to act like Nerevar today." It needs extreme amounts of time and effort. Nerevar united the warring ashlanders and house Dunmer, which was no small feat. Doing the same thing in his name makes you Nerevar 2.0, allowing you to do as he was trying to do (get rid of the Heart) with the benefit of time and yet further experience.
EDIT: A better example would be, Pelinal didn't walk out one day and say "I think I'll pretend to be Shor to become him." He fought for humans against elves, and because that was as Shor did, he became a Shezzarine. This isn't something that gets intentionally tried, it just sort of happens. But like the whole "human belief influences gods" discussion from a while back, it takes more than just saying it.