Set dressing and props serve several purposes:
ANECDOTE ALERT: One time, while shooting a television series episode (murder mystery), a plot hurdle arose. A main character was meant to find her lost diary, which had been absconded for malicious purpose. Filming halted, while on set the director fretted about how to get to that discovery. I discreetly approached him and mentioned that there was a letter opener in the desk drawer (set dressing), and that we had postal mail in the prop truck. I coyly suggested that the character could have just entered the house with said mail and gone to the desk drawer for said letter opener, when viola! the stolen diary. The director was well pleased with this idea, and filming continued.
A few tips:
ANECDOTE ALERT: Once on the set of "Matlock", Andy Griffith ate scrambled eggs for many takes -- even though he hated eggs. Method, man.
ANECDOTE ALERT: On a feature film, we were shooting a fight scene in a scientific laboratory, where the floor was gray. Even after a single take, the floor was filthy dirty. For continuity, it needed to be pristine at the beginning of each take. (Sadly, this was noticed in dailies, and we had to re-shoot scenes.) Problem was that the set floor had merely been painted gray with water-based paint. Cleaning it with water wouldn't work, and there wasn't time to repaint it and let it dry. I used a light sandpaper to remove as much dirt as possible, and used a set of grayscale chalks (garden variety art supplies) to touch up nicks and scratches. Sharpies are great for touch ups, too. Have on hand at least one of every color you can find, including the metallic colors.