This is the 4009th most frequent Chinese word.
充当
To serve as; to act as.
充当 here indicates 'to function as or be used in place of'.
在紧急情况下，门可以充当桌子使用。
In emergencies, a door can serve as a table.
充当 here means 'to serve as or act in the capacity of'.
在这个电影中，她充当了重要的角色。
In this movie, she played an important role.
充当 in this context means 'to take on a formal role or position'.