step in
step in {v.}
1. To go inside for a quick visit.
It was a cold night, and when the policeman passed, we invited him to step in for a cup of coffee.
2. To begin to take part in a continuing action or discussion, especially without being asked.
When the dogs began to fight, John stepped in to stop it before they were hurt.
When Bill had done as much as he was able to on his model plane, his father stepped in to help him.
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Source: A Dictionary of American Idioms