The "Succession" star unleashed a rapid-fire tirade at London's HistFest this week ... slamming JP's 2023 portrayal of the historical figure as downright awful -- even suggesting he would've done the role more justice.

Brian says, "Terrible. It's terrible. A truly terrible performance by Joaquin Phoenix. It really is appalling. I don’t know what he was thinking. I think it’s totally his fault and I don’t think Ridley Scott helps him."

BC adds, "I would have played it a lot better than Joaquin Phoenix, I tell you that. You can say it’s good drama. No -- it's lies."

Brian even played on Joaquin's name to underscore his point, amusing, "I think he's well named. Joaquin ... wackeen ... wacky. It's a sort of wacky performance."

Basically, according to Brian, critics are lying about calling "Napoleon" a solid drama -- even though it did snag 3 Oscar noms, though none were for Best Actor or Best Director.

True to form, Brian doesn't shy away from speaking his mind -- he's famously taken swings at Johnny Depp and even his "Succession" costar Jeremy Strong.