The same two letters, in the same, consecutive order, will change each of the following words into new ones.

MILE
RUST
SCENE
PRATE
SERVE

HINT;
The missing letters precede two of the words and are inserted into the other three.

Answer:

The letters are OB:

MOBILE
ROBUST
OBSCENE
PROBATE
OBSERVE

Credit: LA Times Crossword
Sylvia Bursztyn and Barry Tunick
OB SESSION