If you mean the missions from random people like capturing/killing a thief, returning a horse, help one save his wife, e.t.c., you can just ignore it and it will eventually say mission failed but you won't lose anything. For the main storyline missions, you can maybe suicide, then it'll say retry from checkpoint or exit. You can also kill your own friends or let the enemy run away when they're not supposed to and it'll say mission failed with options retry from checkpoint or exit.
You can do missions for Bonnie MacFarlane in Red Dead Redemption, that is all.
do your missions
Do your missions.
yes
No, in undead nightmare you basically kill zombies and do zombie missions
you have to do missions
on the storyline there is 51 missions and 18 side missions
i have both and i believe red dead redemption is a lot better as on mafia a lot of the missions are very similar whereas there is a lot more variety in red dead redemption. also red dead redemption has a much better and interactive environment
do the missions which it says in your journal
there is no cheat for it, you must do the missions for it in your journal
you must do more missions until it unlocks
If you want to be able to do this you need to complete the story missions on Red Dead Redemption and eventually you will unlock the higher levels of dead eye which allow you to use the X's.