GTA 5 Mission Walkthrough: Chop
What starts as a simple walking the dog mission, quickly escalates into one of Lamar's schemes to kidnap an opposing gang member.
After rolling up in a white van, you'll have to chase him down, though, be mindful, you're in a van, and he's on a motorcycle. You'll eventually chase him down into the trainyard. At this point, you'll have the opportunity to play as Chop briefly, to sniff out the gang members scent while he's hiding in one of the trains.