Learn German: Der Regisseur erklärt die Rolle, bevor wir spielen. - The director explains the role before we act.

The word bevor (before) is a subordinating conjunction. In German, these types of connecting words force the conjugated verb in their part of the sentence (the subordinate clause) to go to the very end.

Is the comma before bevor strictly necessary?

Yes! Unlike in English, German has strict comma rules. You must always use a comma to separate the main clause (Der Regisseur erklärt die Rolle) from a subordinate clause introduced by words like bevor.

Doesn't spielen usually mean 'to play'? Why is it translated as 'act' here?

You are right, spielen primarily means 'to play'. However, in the context of theater or film, it is the standard verb for 'to act', much like how an actor 'plays a role' in English.

What happens to the word order if I want to start the sentence with Bevor wir spielen?

You can definitely start with the bevor clause! If you do, that entire clause counts as the first position of the main sentence. Since the main verb must always come second, the next word has to be the verb erklärt. It becomes: Bevor wir spielen, erklärt der Regisseur die Rolle.