Actors who starred in movies to get credible, but falied...

Sylvester Stallone in Copland.

How did Andy Kaufman alienate his audience?

While this is your thread to do with as you please, that is a completely different topic and could be a major hyjack and run a lot longer than the original. My suggestion would be to open a second thread on that topic.

But in regard to the original question, may I suggest Dick Van Dyke with The Runner Stumbles and The Entertainer.

TV

Julia Roberts in the HORRIFIC Mary Reilly.

Britney Spears

Tom Selleck. He’s never been able to shed Magnum P.I. no matter what he’s tried.

And then again in The Majestic.

Michael Jordan proved his comedic prowess in Space Jam.

Vanilla Ice knocked us dead with his heartwrought portrayal in Cool as Ice.

Brian Bosworth left no doubt as to the 90s next action star in Stone Cold.

I’d have to cite Shelley Long, who went from the female lead of a top rated TV show to movie oblivion.

When I saw the title of this thread, this was the first naem that popped into my head.

Maybe we can add Jennifer Anniston to this list if her turn in the Good Girl doesn’t pan out. Too soon to tell. Or Adam Sandler in Punch Drunk Love (where he was pretty good).