So I think this question started out solidly but recently mutated due to the discussion in the comments into a bad question.
How do you motivate players to be content with randomly selected roles that are often complex to play and differ drastically in playstyle (AND to stick around till the end of game)?
I think this bit here is a good solid question, I interpreted it as "How do I engage and retain players who are assigned random roles that are involved enough to take multiple playthroughs to learn in a Mafia style game?". Mafia style games are quite common and popular so there is likely to be documented opinions and experiences from industry experts additionally there are probably also psychological studies that can be used to form answers. Additionally this seems to be the question being answered by the more highly voted answers.
However the OP summarized the question in a comment that read as quoted below.
The question can be really boiled down to "how to promoting interaction between dead vs alive players while keeping the game balanced."
I don't like this question. It can not be generalized to other games in the same genre and requires an depth knowledge of the OPs game.
So question is, do other people agree with me? And if so should the question be closed and re-asked or edited to what I believe the original question was?
Either way I think the question should be edited so that it is general enough where a link to the OP's game is not required and if that's impossible closed.