Right Turn signal light not lit

Got a 2003 Pontiac Grand Prix GT 3800 with 112k on it. My right turn signal bulb only works when the turn signal is engaged,but is not all the time like the other side. Im replacing the bulb today but....what do you think would cause it to light only with the turn signal on?

I think you'll find it working after changing the bulb. Some bulbs have a dual purpose, and with that, two filaments in them. One could burn out while the other one still works.