Dwayne Johnson signed up for 'G.I. Joe: Retaliation' to improve the film series.

The 39-year-old star - who also wrestles in the WWE as The Rock - has joined the franchise as Roadblock, and he thought there was ''room for improvement'' in the movie series after watching 2009's 'G.I. Joe: Rise of the Cobra'.

He said: ''I was even more than 'Yeah I'm in.' But also too, you wanna be a little bit poised with your excitement at first. When I first heard it, the idea of starring in 'G.I. Joe', first of all you get a second crack at something that is - the franchise is that big, the title is that big, the first one did well [but there was] room for improvement.

''We get a second crack at bat with this; I wanted to grab this by the throat and not let go. The moment I locked in, we all agreed, I'm in.''

Dwayne trained his ''ass off'' to get in shape for the action movie - which sees the G.I. Joe special mission unit go up against a spy who assassinates most of their members - because he was determined to ''pay honour and respect'' to the US military.

He added to Collider.com: ''I trained my ass off for this role, wanted to become Roadblock, wanted to pay homage to the mythology of G.I. Joe, and not only that but then pay honour and respect to our military in a real way where it's gritty and rooted and grounded and boots to the ground, and we're not shooting off lasers and s**t; these are real rounds.

''We wanted to pay homage to and elevate the fantastical side of G.I. Joe with Snake Eyes and Storm Shadow and the incredible ninjas that we have. It was a great blend of worlds, Collision of worlds, and when they came to me with the idea of starring in it was like, there's the throat, there's the grab, I'm in.''