Yup: Rambo can write. After bit parts in '70s films like Bananas and Death Race 2000 (and a porno), Sylvester Stallone made a name for himself in 1976. And that name was Rocky Balboa.
A boxing fan himself, Stallone created the character of Rocky, a hard-hitting amateur boxer with a heart of gold, and his opportunity to have one shot at success.
A lot of '70s films featured dark, violent anti-heroes—think Travis Bickle in Taxi Driver or Popeye Doyle in The French Connection. And that's how Stallone originally envisioned it. But Stallone's wife hated the nastiness, so Stallone slyly rewrote it, and made Rocky into the kind-hearted soul with fists of concrete who we know today. (Source)
Rocky's rags-to-riches story came true for Stallone, who went on to write and star in four more Rocky films, numerous Rambo films, the Expendables franchise, and the classic comedy Stop! Or My Mom Will Shoot (1992) with Estelle Getty.