Often, different lines of one verse are recorded separately then mixed together by their DJ so the artist can focus on making each line powerful and intense.
Also, most rappers tend to record their verses twice and overlap to give the song more atmosphere and feeling. Good examples of using this technique include Prayer by Rick Ross and Crooked N**** Too by 2pac. Snoop also uses this effect frequently and effectively
In live shows, rappers use another MC to emphasize certain phrases in their songs, commonly the last few words of a bar.
Snoop: I'm in the club, with the snub, gettin' love...
The Game:..........Club!..............Snub!.Gettin' Love!
This happens since on the album version the artist emphasizes his lines himself, although it often goes unnoticed even though it plays a big role in rap songs.
Adding shouts or related calls in a song (think of Lil Jon's hype bursts of "Hey!" or "What!?" or "Yeah!!!")
gives a song more feeling and obviously enthusiasm.
Hope this was useful!