Do thrust reversers only have idle and full modes or are there intermediate settings?
My experience is on the CRJs but most airliners are similar.  Normally you can work reverse thrust variably between idle and max reverse.  The reverser levers will have a stowed and locked position (all the way down), and there will be a latching device you operate with your fingers to pull them up from stowed.  

From there you end up at an idle reverse position (which may have a physical detent you can feel), and there is a range of movement above idle reverse going up to maximum reverse.  You can vary the reverse thrust between idle and max as you wish, although most pilots will just go from stowed to idle to max, then back to idle then stowed on a landing.  

One technique issue on using reversers is if you slam the reverser levers back down to fully stowed really quickly, the reverser devices on the engine stow while the fan is still running at high rpm and on some engines you will get a little shot of forward thrust at the end of the landing roll.  It feels like you let off the brakes at bit for a couple seconds.  Once you get a bit of experience, you learn to ease them off to idle reverse and wait at idle a few seconds before stowing.

Reverse thrust is not taken credit for in landing distance calculations, so their use is optional by the landing pilot.  Some airlines may prefer a bit faster brake and tire wear to wear and tear on the reverser system, based on a relative cost analysis, and encourage crews to avoid reverser use unless they need them.

Some pilots will plan not to use reverse but will want it ready just in case, so they will unstow on landing but keep the reverser levers at idle.  This practice is frowned upon by the maintenance types because it counts as an operating cycle of the reverser system (impacting long term maintenance) even though reverse wasn't used to help slow down.