waiting in the wings

be waiting in the wings {usually in continuous tenses}
(be ready to be used or employed instead of someone or something else) — ждать, поджидать своего часа; поджидать удобного случая, выжидать удобный момент (букв.: находиться за кулисами в ожидании своего выхода на сцену)

If someone or something is waiting in the wings, they are not yet active or important, but are ready or likely to be so soon.

Example 1: The rumour is that Jim will be sacked and Ann is waiting in the wings to take over as manager.
Example 2: Waiting in the wings, there was a young colleague of his. Charles wanted his protégé in and I happened to be in the way.

See also
[bide one’s time]
In the theatre, the wings are the sides of the stage which cannot be seen by the people watching the play, where actors wait until it is their turn to walk on to the stage.