SCENE: You see a drug deal go down between a well-known drug dealer and a shit that you know from school/work. Your dad is a cop and could really use a boost in his career (i.e. catching the drug dealer). Would you tell you're dad that your willing to testify, and have him catch the drug dealer? Remember, this is a well-known drug dealer, and he has friends and junkies who need their drugs who will put a bounty on your head if you do this, but your dad is desperate, and could really use a raise by way of promotion, and he has a good chance at being promoted if he catches this thug. Would you put you and your dad's lives on the line, to boost your dad's career and bring in some much needed cash?
I'd want to help my dad, and maybe by way of a sting operation, if they can locate the fugitive, there would be less worry, and I wouldn't have to testify, and if my dad was credited with the brainwork, but not the arrest, he would have less to worry about. Also, although I would prefer not to, if there was a reward my dad could bust him, I could testify, and then we join the Witness Protection Program (or Witness Relocation Program) and relocate with new identities. (the reason I say if there's a reward is because if there was none there would be no financial gain throught relocating as my dad coudl no longer help the police station, thus he would not receive income from them anymore, let alone a raise) (REMEMBER: Pretend your dad is a cop, mine isn't, so just pretend!)