Nobody You'd Know:

Self tapping screws installed with an electric drill have worked for me.
They're usually located in the same area of the store as drywall screws. Be
sure to get them long enough to go deep into the risers.

Do you suggest going into the risers from below the stairs or from above? Or
both?

Screws are better, to be sure, but not as easily hidden. Sure you can
countersink and dowel or putty over, but hiding a finishing nail driven from
above is much easier than a screw in the same place.

And driving into stringers (they're beefier and easier to hit, yes?) is
easier than into risers. Probably should run into both.

Thanks,
--
John English