a lot of it depends on what sort of situation you plan on putting them in. are you planning on putting them in a game where they're going to be expected to use their smarts on a regular basis for plot purposes (like, say, a murdergame)? in that case, i honestly would advise against picking up a character who is significantly smarter than you are.

but if it's a game where they wouldn't need to be using their intelligence all the time, then you can get away with a lot by just doing some basic research into the specific things they would be knowledgeable about, as other people in the thread have said!