1. The same three letters, in the same order, will complete the words below:

SP _ _ _
CR _ _ _
DR _ _ _
SCR _ _ _

2. The same three letters, in the same order, will complete the words below:

C _ _ _ L
G _ _ _ L
C _ _ _ ET
G _ _ _ LING

Answers:

1. The letters (and word) are AWL:

SPRAWL
CRAWL
DRAWL
SCRAWL 

2. The letters (and word) are RUE:

CRUEL
GRUEL
CRUET
GRUELING

Credit: LA Times Crossword
Sylvia Burzstyn and Barry Tunick
WITHOUT REGRET  (Idea)