One can believe true what one knows is false but can one disbelieve what one knows is true?

I suppose one can profess belief or disbelief in anything. However, knowledge is a subset (and a greater conviction of) belief. Therefore, one cannot earnestly believe things they know to be false or earnestly disbelieve things they know to be true.

Well, I don't think one can be seriously earnest and at the same time believe a fictional story true for one's amusement.

Then doesn't this contradict your previous statement that..."One can believe true what one knows is false"

??

If it is on the internet, it must be true.

If tommy thinks something is true it must be about a different thread or theme.
If it is about the current topic it is false.

Just an option after many year here , but may or may not express my actual view of things.

Suspension of disbelief is the avoidance—often described as willing—of critical thinking and logic in understanding something that is unreal or impossible in reality, such as something in a work of speculative fiction, in order to believe it for the sake of enjoying its narrative. -Wikipedia

I think the above is a belief but I do not think it is an earnest or serious belief: I think it is an entirely irrational belief but not insane. It is normal to believe let us say Superman is real when watching a Superman movie.

On the other hand, I think if one rationally decides that something is true I don’t think one can then deny it, or pretend it is false, without appearing insane.

I say I think when I don't know and that is why I ask.

Ah tommy, you original post did not limit the view to theater.
Yes, we can play all sort of game with theater and pretend venues and fantasy games.

In real life, believed things and known things are different, but really confused by the masses.
Truth is not popularity contest. If you know the truth of something, you may pretend at a different view but that is not 'believing', just a lie.

If you only believe something there is no truth, just a level of certainty changeable to conviction or certitude by experience and choice.

Sadly, most folks prefer believed things and conviction over certitude or knowing.

You can pretend either and lie about it. Fooling yourself is the real problem.

Is one insane claim a known falsehood to be fact? No - it only means you are politician or marketer/preacher

“The Party told you to reject the evidence of your eyes and ears. It was their final, most essential command.”

George Orwell, 1984

Happens all the time in human society.

It has become a standard prerequisite delusion amongst masses.

For example.

Scientists who truly believe in their organized faith systems.

Systems that go against their scientific knowkedge that are evidence-based.

Scientists, I think, ought to stick to attempting to explain the physical real world and stop fantasying that science can explain, or prove or disprove the existence of the non-physical phenomenal world, which we all know exists.

There you go, you just answered your own question that started this thread.