Each knew their roles...

Tang, that depends. Usu we say "his role" but we can imagine a situation where each participant has more than one role, in which case it's "his roles," unless you're sensitive about "their" sex, in which case "his/her" role or roles

However "their" if often used precisely to avoid the problem so here we oft do use "their roles" convention

If each person knows one role, then the non-gender 'their' takes the singular just as 'his' or 'her' would, so it would be 'role'.

True Tes, gotcha, except of course....

Except of course what?

When each participant has more'n one role

Forgive, Tes

That is quite right.