Fetching details from the dictionary...
to turn completely into; to become completely
彼は悪役に完全に成り切った。
He completely became the villain.
少女はプリンセスに成り切っている。
The girl is fully absorbed in being a princess.
役者として別の人物に成り切るのが難しい。
It's hard as an actor to fully become a different person.