What kind of stairs are you talking about? Can you post photos?
We purchased a "fixer upper" and it sat empty for a little more than a year. The smell was horrific, so we ripped up the carpet everywhere. There was mold from urine, so don't judge!! :) What I am trying to do is make them a little less noisy, we have been sleeping in the room right next to the staircase downstairs. My husband works nights, so tries to sleep during the day! We don't have definite plans so its just a for now fix?? I put some great stuff foam spray in between a lot of them, but there has got to be something easier? sorry the pictures are not the best!
The creaks are due to wood movement. Spray foam is best for sealing air gaps and is not "structural".
I would sink some screws down through the nose of the treads into the risers. Putty the holes and paint the stairs and risers so it is not visible.
To a lesser fix some poly based construction adhesive could be applied and forced into the gaps from behind, and then blind screwed at an angle form behind as well. This would maintain the unblemished look from above...it these were stain grade hardwood treads
Glue 45deg blocks underneath and add wedges
Work from under the stairs. Use a 2"x2" board the length of the stairs. Spread wood glue on two adjoining sides. One for the riser and one for the tread. Screw the board to the riser first with 2" deck screws leaving about an 1/8" gap between the board and tread - about the thickness of four credit cards. Then screw the board to the tread with 2" deck screws. Deck screws have a coarser thread and will bite into the tread pulling it down tight to the riser. Predrilling the board will make the job easier. Hope this helps and good luck with your project.