Choosing the right instruments for a particular song is one of the most difficult, yet fun, parts of the producer's job. Try to imagine “Margaritaville” without the steel drums or “Turn the Page” without the sax part. Even better, try to imagine “Margaritaville” with sax and “Turn the Page” with steel drums. Yikes! Just as much as the groove or style, instrument choice can influence the overall feel of a production and help position a song for success in a given market.
Certain instruments lend themselves to particular genres of music. When you hear a banjo, what do you think of? How about pedal steel, turntable scratches, violin, conga drums, or distorted electric guitar? Each of these instruments instantly conjures up a specific kind of music. Knowing this can help you mix and match instruments for a particular flavor or feel.
Some instruments work well for several styles of music. A Hammond B-3 organ can be used for a Bach Toccata or a classic rock song like “Freebird.” An accordion might bring to mind polka night at the bowling alley, but it's also important in zydeco, Mexican country music, and some of John Mellencamp's roots rock. There are no rules concerning what instruments can't be used for a given style. You might use mandolin on a rock song (like Rod Stuart's “Maggie May”) or synthesizer on a country song (like Shania Twain's “Man, I Feel Like a Woman”). Classical-style string arrangements have been used in many different genres to “sweeten things up” and help a song's pop crossover appeal. This is where a producer has to go with instinct and hope he or she will make the right decision.
Familiarizing yourself with the differences of the major instrument brands can be a big help. Want a twangy country guitar? You probably need a Tele through a Twin. Classic rock? Try a Les Paul through a Marshall. Gritty blues? It might be a Strat or an ES-335. Ask musicians about different instruments.
If you have enough tracks and enough money, you can experiment with the instrumentation, but don't get too carried away: Your ultimate job as a producer is to choose the combination of instruments that will best accentuate the song.