I'll toss out that years ago I had an S&W 14-2-a very similar gun to the model 15(both K-frame adjustable sight 38 special revolvers), although the 14 was REALLY intended for target/bullseye use more so than the 15.
The thing scared me the first time I took it to the range. I'm no gunsmith(just a watchmaker who has taken a perverse pleasure in fixing S&W and Colt revolvers at times), and it's been years since I had an S&W apart, but there are 3 springs-the mainspring, the trigger rebound spring, and the hammer spring. The mainspring is a leaf spring, and the other two coil springs. There's some delicate interplay between the three to set the trigger bevavior, and often times people who do a "trigger job" will only mess with the mainspring. It's the biggest, most visible, and easiest to change(the first time you try to fit a trigger rebound spring, you'll think having 3 hands isn't enough). Putting a light mainspring in can also make firing unreliable, as the hammer may not hit the primer hard enough.
In any case, though, the trigger was so light on this particular gun that I fired it a few times just putting my finger in the trigger guard. Fortunately I was following the basic rules and had it pointed down range and at the target(even if not exactly with the sights aligned). The first time I had it at the range I was using my target handloads with Federal primers and it fired about 90% of the time, but another trip with some harder CCI and Remingtons had it down to about 50%.
I forget what exactly I ended up doing, but I think I bought a full Wolfe 3 spring kit and just went across the board lighter than stock on all three springs. Again, it's been years since I did much with this, but I think the stock mainspring is 22lbs and rebound is 14lbs, and I believe I went to 12lbs on rebound and 16lbs on the mainspring. That gave me a light and crisp but manageable trigger, and I went to the lower limit of what mainspring will give reliable operation.
With that said, if you don't know what you're looking at, it's worth having someone who does take a look. Trying to remove the sideplate without the correct screwdriver can mangle the screws, and if you don't know the "trick" for getting it loose you'll likely bend it. There are tricks to lighten the mainspring without changing it, and someone use to working with these can spot them(and do it right by actually installing a proper spring).