on one's best ／good behavior
behaving right行为规矩
While his father was around，the boy was on his good behavior， but when the father left， the boy soon got into mischief．父亲在场时，这孩子规规矩矩，但父亲一走，他马上就调皮起来。
The naughty boy is on his best behavior today．今天这个调皮鬼表现很好。