does the crew agree collectively to pick such a leader, or is the leader forced upon them by threat of violence?

That's related to, but not fundamental to the definition of authority, whose core point is enforcement via class violence (IE either capitalist authority or proletarian authority). You can have a completely democratic selection process, and the enforcement of that decision is still "authoritarian", unless you want to allow everyone to break the decisions that are reached.

Marxists already have this collective democratic process, its called democratic centralism, which takes many different forms depending on the situation, but is essentially "diversity in discussion, unity in action". One of the first practictioners of it was Mao, who impressed that individual red army units during the Chinese civil war should select their own officers.

Once decisions are selected, they are binding upon the members. I can tell you from experience that "anti-authoritarian" types consider even democratic rulings "oppressive", and that they feel free to break them since that imposes on their individual freedom.

What if they want a different leader, are they able to choose a new one?

That also is related to, but not fundamental, to the question of authority. Recall exists(ed) not only in communist countries, but even in liberal dictatorships, where recall does absolutely nothing to hinder the authority of the capitalist class.

If anti-authoritarianism to you (like many anarchists) means the freedom to disobey (even democratic rulings), then its no wonder that every single historical anarchist attempt has lasted less long than it took most of us to get through highschool.

The only way for a revolution to survive is for proletarian authority to be even more organized and more disciplined than its capitalist opponents.