подменить(кого-либо – for someone; на работе и т.п.: I couldn't find anyone to step in for me so that I could take a break from my caregiving duties4uzhoj)
вступить(в значении "вмешаться", "сказать своё слово": Given some of the court decisions on this, its probably time for Congress to step in and clarify some of these issues.4uzhoj)
начинать работу в студии(начинать работу в студии по записи музыкального материала (Disturbed stepped into the studio after stepping off of the road and began work on a new disc – All Music Guide). sankozh)
ходить на сторону(It wasn't the first time he'd been accused of stepping out, but the high-profile nature of his relationship with actress Sienna Miller made Law's cuckolding that much more damaging to his career.)
проявлять инициативу(в отношениях; делать шаги навстречу. "No longer will you cling to relationships for the emotional security if the other person does not truly step up to the plate and treat you... " OLGA P.)
