Roles have abilities on their own and can work on someone who would otherwise be Vanilla.
As convention, modifiers are typically adjectives while roles are typically nouns.
Macho and Bulletproof are roles despite being adjectives, as no one says that a Macho Townie or Bulletproof Townie isn't a role card that makes sense. Ascetic is a role, and it works as either a noun or adjective.
There are also derived roles such as Finder and Enabler. They need a role (or modifier) attached but are not modifiers, as these roles refer to a role (or modifier) but aren't actually the role they refer to.
Ninja and Strongman are unusual in that they both refer to a role and a modifier, though the role version of Ninja and Strongman modify factional abilities. Notably, the modifier version of Strongman is still not Normal yet.
If we leave what is allowed in Normal Games, we start to see modifiers that make sense on a otherwise Vanilla Townie, such as Superstar and Flying. These do change how roles work but have abilities on their own, and technically are
both
roles and modifiers.Rule of thumb for determining if something is a role or modifier: Try attaching that role or modifier to a Vanilla Townie. If the resulting role card makes sense, you likely have a role. If it doesn't, you likely have a modifier.