The same two letters, in the same, consecutive order, can be inserted into the words below to form new ones. Can you determine the two letters and the new words?

DEAL
GEE
FISH
DING
CASTER

HINT:
The two letters are inserted after the second letter of each clue word. 

Answers:

The letters NI can be inserted into each word to form a new one:

DENIAL
GENIE
FINISH
DINING
CANISTER 

Credit:
LA Times Crossword
March 13, 2016
Don Gagliardo and C.C. Burnikel
PLUGGED NICKEL

Note: Ni is the chemical symbol for nickel.