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

FILE
PILE
TILE

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

ID
FEE
INK
BOW

Answers:

1. The added letters are CK:

FICKLE
PICKLE
TICKLE

2. The four letters are LING:

IDLING
FEELING
INKLING
BOWLING

Credits:
#1: LA Times Crossword
December 4, 2009
Jack McInturff

#2: LA Times Crossword
November 15, 2009
Mike Torch
LING-UISTICS