How To Repair A Loose Newel Post

In this video, This Old House general contractor Tom Silva shows how to bolster a wobbly post with a single screw.

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Wedge a temporary wood brace between the nearest wall and newel post; the brace holds the post tight against the stair tread.
Use a drill/driver and ¾-inch-diameter spade bit to drill a ¼-inch-deep counterbore hole into the post. Be sure the hole aligns perfectly with the stair tread against the opposite side of the post.
Drill a screw-shank clearance hole into the post with a standard-length drill bit. Hold a layout square against the post and use it as a visual guide to ensure you’re drilling perfectly level and straight.
Switch to an extra-long bit of the same diameter as the screw-shank-clearance bit, and drill a hole completely through the post and just barely into the end of the stair tread.
Next, drill a screw-pilot hole into the stair tread using an extra-long bit that’s slightly smaller than the diameter of the screw.
Drive the long screw through the post and into the end of the stair tread.
Remove the temporary brace.
Use a putty knife to apply wood putty over the screw head. Allow the putty to dry, then apply more putty to overfill the hole.
Once the putty has dried, sand lightly, then prime and paint the putty.