step into the breach › to do someone else's work when they are unable to do it: Bill's illness meant that Kathy had to step into the breach.
step on it informal › used to tell someone to drive faster or to hurry: Could you step on it? I'm late. If we want to get these orders out on time, we're going to have to step on it.
step out of line › to behave in a way that is unacceptable or not expected: If he steps out of line, he could be fired.