step in

verb

1. To act as a replacement or substitute.

   “McCoist unexpectedly ushered back a defender of his own with Kirk Broadfoot taking over from Steven Whittaker. There was, of course, another change, Kyle Bartley stepping in at centre-half to replace suspended Dorin Goian.”

2. To get involved; to act.

   “HM Chief Inspector of Railways has stepped in to speed up work to resolve incidents of overspeeding, having been concerned that progress on the issue has been too slow.”