The same two letters, in the same, consecutive order, will change each word below into a new one.

ROBS
MUTE
SISTER
FANCIER
BULLETS

HINT:
JURIES could be added to the list.

Answers:

The letters IN can be inserted into each word to make a new one:

ROBINS
MINUTE
SINISTER
FINANCIER
BULLETINS
(INJURIES)

Credit: LA Times Crossword
Sylvia Bursztyn and Barry Tunick
SOLUTION WITHIN