In my plane? I'll run the checklist, which is parsed out so that you pull the handle and then hack 30 seconds, then if that doesn't put the fire warning out you blow one bottle and hack another 30 seconds, and if THAT doesn't do it, then you blow the second bottle. If you've REALLY got a fire on your hands then the worst thing that'll happen is the engine burns off the airframe and your new emergency is trying to figure out how much civil liability is associated with a Donnie Darko style accident with a flaming engine falling into somebody's house.
In a 421? I don't know, I don't have any 421 time, but I've got some in the 99, which had some problems with inaccurate fire warnings. In that SPECIFIC AIRCRAFT, if you got direct sunlight on some of the sensors or (oddly enough), a bunch of rain (if I remember correctly), it could trigger a fire warning. The guidance was to take a look outside, check out your instruments, consider if you're flying into the sun and/or through a rain cloud, and then make a decision. Compounding this decision, in many situations turning a few degrees (if in the sun) would usually turn off the warning, and/or climbing or descending (out of the rain) would usually turn off the warning. So basically, you had a few things to do prior to simply punching the fire bottle.
Point being? Follow the procedures for your aircraft. Don't assume it's NOT a fire, and in fact I'd assume it IS a fire, but wind the clock and run the procedures FOR YOUR AIRCRAFT. There are going to be very few truisms for emergency procedures like this, so don't rely on what you do in other aircraft. I know that in my aircraft, the checklist is designed so that you are required to wait certain periods of time before really going balls out on the problem. I know in other aircraft, that may not be the case.
But know your aircraft, and know your procedures.