Movie Stars that can handle musical instrument well

Not enough to be “triple threats” (act/sing/dance) in show business, they were pretty good instrumentalists as well:

https://youtu.be/baHH2Kqz1SQ

https://youtu.be/7bYvZEpHEMA

And, of course, an Academy award nomination for “Best Actor” makes one a movie star by any standard. He was inarguably a star as an instrumentalist. Some might even argue, the best tenor saxophone player of all time:

https://youtu.be/gIYV5kIdK9Q

Taking the twist on the OP a bit further, star instrumentalists who could handle a little acting:

https://youtu.be/YtjzVThkiFs

It's an old trick to include famous actor for better commercial success. Johnny Depp plays scored few notes on the guitar pretty clean, but still think it's more for commercial success than for the benefit of the gig.

I know that Colin Firth learned to play the guitar as a child, but later he was banned from this instrument. Instead, he was offered to learn how to play the euphonium - a bass musical wind instrument. Later he played in a rock band, but it's just not very clear on which instrument.

The Bacon brothers