Kevin O'Leary is explaining how the hell he ended up starring alongside Timothée Chalamet on the big screen ... and it sounds like he may have been typecast.

Mr. Wonderful joined us on "TMZ Live" and we asked him how he went from "Shark Tank" to the upcoming Chalamet movie "Marty Supreme."

The trailer dropped Wednesday and lots of folks were left scratching their heads after Kevin got a lot of facetime.

Kevin says the director and the writer came to him looking for a "real a***hole" to play the fictional Milton Rockwell, one of the wealthiest men in America in 1952, when the film is set. Getty

O'Leary tells us his character is married to a movie star played by Gwyneth Paltrow ... and Timmy's Marty Supreme is a real hustler who needs Rockwell's plane and money to make his

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