Leadership in an ensemble can take many forms, ranging by degrees from a detail oriented clockmaker, who finely tunes and controls every moment of performance, and an ensemble member who does a count off to start the performance and then fades back into the ensemble.
The range does not include no leader. An ensemble needs someone to start things. A duet can just start together, and even a trio can make eye contact and act as one. Any larger, and someone has to be in charge. You need someone to start things, at the very least.
Ensembles don’t simply self-organize, and they can’t go on without leadership. No matter how small, they require someone to lead.
Knowing how to be that leader, and recognizing what amount of leadership to provide: there’s the trick.