Well, Brutus participating in the assassination isn't just theater; there's no ancient source that contests his role that I know of. Admittedly the narrative sources for the assassination are 2nd century or later, so there's room for a fabricated tradition they all draw on. There's no evidence of that fabrication though; as I said there's no tradition that says Brutus wasn't involved.

Can I ask why this question came up? I feel like there's more behind this than just whether Shakespeare was historical fiction and I would be able to give a fuller answer if I knew the background.