My not especially deep theory of Richard's disastrous gaffe
Still just about counts as a spoiler I think
Here's what I think caused Richard E Grant's disastrous "I'm a traitor" gaffe.
He was in "after party" mode.
His brain was in that mode of "well, I've done my acting tonight and that's over with so now I can just relax with a drink with the other thesps"
Presumably for most of his acting career there have always been this division between when you're acting, when you're "on stage", you're performing you keep up the facade for the punters, etc, and then the other times when you're relaxing with your fellow cast and crew after the performance is over, often with a drink in a nice bar or (probably) castle. At that point you - literally - drop the act and chat about how the play or whatever is going.
He just forgot in that moment that this game doesn't work like that.