So you create a function around the message. It's not uncommon at all to have something like (pseudocode)

myActorModule.increment(ref, amount) -> ref ! {increment, amount}

myActorModule.syncIncrement(ref, amount) -> ref ! {increment, amount, self()} receive -> ok timeout 1000.

You can build out the message passing interface to an actor -as- functions, and even supply sync ones if you want. The actor implementation can be completely opaque to the caller; just the exported interface functions are defined, exported, and supported.