There's also an issue where Hewitt's actors were more like Erlang processes, i.e. unlike Scheme closures, they could run independently of each other. Maybe call/cc can simulate something like that. I remember the footnote in SICP claiming that Scheme was developed partly to understand what Hewitt was talking about, but I think that might not have been serious. It could be worth trying to talk to Steele or Sussman about this history.