Commonly hiring a private detective means you must be prepared to pay daily or hourly rates with expenses on top, if money is no object a private investigator is your own personal secret service to do your bidding.
If you will interview them personally then you will find out whether they will be able to solve your case or not. If the detective selected denies providing you proper agreement or contract, then remember they are not the correct detective you are looking for. So, simply look for some other perfect detective on which you can rely.
Hiring a private detective with the intention of following someone you suspect of adultery for instance is a very logical reason, it’s hardly paranoia when you have a distinct feeling your partner is cheating on you but you cannot find the proof. Hiring a private detective isn’t entirely rooted in pushing back against people conspiring against you either; it can be done as a tool of love in some cases.