In Chris Farnum's article What an IA Should Know About Prototypes for User Testing, the issue of the 'degree of fidelity' is addressed...
Usability practitioners like Barbara Datz-Kauffold and Shawn Lawton Henry are champions for low fidelity --the sketchier the better! Meanwhile, Jack Hakim and Tom Spitzer advocate a medium- to high-fidelity approach that gives users a closer approximation of a finished version. You'll want to make a decision about the right approach for you based on the needs of your project.
I'll add in my two cents and say that the higher the fidelity the better, within the constraint of the cost of the prototype. As in, the more you can make the user forget about the medium of the prototype, and thus the more you can make them focus on what's important, the better. In my experience, clients, customers and users (often, all the same person/people) have a hard time getting around anything in the prototype that doesn't make sense. I have often had to fully immerse the user in the prototype by including relevant and current data in a prototype.
Again, when the user/client was able to 'suspend their disbelief' (a term often used within the scope of watching a movie) due to a high fidelity prototype, they were more apt to comment on the interaction design and usability of the prototype. This point is made is made in Farnum's article, and I'm offering a concrete example.
Unfortunately, the higher the 'fidelity' of the prototype, the more it is going to cost, in terms of time and money (and time is money).
To go thru the effort of creating a prototype that is very similar to the envisioned finished product means you need to get real data, real information, real design and real effort involved. None of that is cheap, and will often dictate how realistic the prototype can be made. I my opinion, prototyping is like buying a computer. Figure out how much cash[time] you have to spend and buy the best thing you can afford.