Rappers have backup lyrics running often. Either that or they perform with another person to finish off some lines which are pretty hard to complete live because of shortness of breath etc. Eminem has done this at many major award shows I have seen. Generally he has his D12 partner, Proof, finishing the lines for him.
Eminem is definitly one of the better live rappers out there though, but with the way some of his songs are structured it is virtually impossible to do live and the same pace without losing breath.