stand in for someone—act as a deputy or replacement for a person:
- The actor might get injured so someone who looks like him should stand in for him in this scene.
stand in with someone—
1. have an understanding or community of interest with smb.:
- Of course if you stood in with the chairman there’d be no difficulty in having your plan accepted.
2. share a cost with a person:
- All the members agreed to stand in with the secretary, who had bought the club supplies with his own money.