Source : WordNet®

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}, {interfere},
           {interpose}]
     2: act as a substitute; "She stood in for the soprano who
        suffered from a cold" [syn: {substitute}, {deputize}, {deputise}]