Usage patterns
〈人〉が役職に就くfor someone to take up a post/position
He is scheduled to take up a new position next month.
〈人〉に役職を任せるto entrust someone with a managerial position
The company entrusted her with an important position.
役職から退くto step down from a post
He stepped down from his position for health reasons.
役職に就いているto be holding/occupying a post
She holds the position of department manager.