'step in' definitions:

Definition of 'step in'

(from WordNet)
verb
Get involved, so as to alter or hinder an action, or through force or threat of force; "Why did the U.S. not intervene earlier in WW II?" [syn: intervene, step in, interfere, interpose]
verb
Act as a substitute; "She stood in for the soprano who suffered from a cold" [syn: substitute, deputize, deputise, step in]

Synonyms of 'step in'

From: Moby Thesaurus