I can think of a really easy way to do this; have his first action scene be him beating the piss out of Killer Croc....as in an actual large lizard man. That should set the tone right out of the gate.
RUMOR: David Tennant Being Looked at for 'Justice League' ? ~ Superhero Movies News | Comic Book Movie News 24/7 David Tennant as Plastic Man? Hmmm...
Warner Bros. Will Announce New DC Comics Movie; Could It Be JUSTICE LEAGUE? "Two new television shows are coming to the CW and Fox, including one based on the Flash and another on a young Batman, and a film series will be announced in the near future, Mr. Tsujihara said. It is expected to include a “Justice League” movie." Finally! We may have to wait for BvS but they are definitely assembling things.