WordNet (r) 3.0 (2006):

step in
    v 1: 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]
    2: act as a substitute; "She stood in for the soprano who
       suffered from a cold" [syn: substitute, deputize,
       deputise, step in]

Moby Thesaurus II by Grady Ward, 1.0:

65 Moby Thesaurus words for "step in":
   act between, admit, arbitrate, bargain, barge in, be admitted,
   break in, breeze in, burst in, bust in, call, come barging in,
   come breezing in, come busting in, come by, come in, come over,
   creep in, cross the threshold, crowd in, drop, drop in, edge in,
   enter, gain admittance, get in, go between, go in, go into,
   have an entree, have an in, hop in, insert, intercede,
   intermediate, interpose, intervene, intrude, irrupt, jam in, judge,
   jump in, look in, make terms, mediate, meet halfway, moderate,
   negotiate, pack in, pop in, press in, push in, put in, referee,
   represent, set foot in, slip in, squeeze in, take in, thrust in,
   treat with, umpire, visit, wedge in, work in