step in

Verb	1.	step in - 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?" Synonyms: intervene, interfere, interpose
	2.	step in - act as a substitute; "She stood in for the soprano who suffered from a cold" Synonyms: deputise, deputize, substitute

horn in, interact, interfere, interlope, interpose, intervene, intrude into, meddle, meddle with, poke into, tamper