Before he put on his superhero outfit, the highest-paid actor in the world was considered a risky bet.

Robert Downey Jr., though he has long been regarded as one of America's finest actors, had two strikes working against him. The first was that while he had talent as proven in films like Chaplin, he was more of an art-house/independent-film actor who was not considered a box office draw. The second -- and this is the big one -- is that his long history of substance abuse made him a risky bet for any employment, let alone becoming the highest-paid working actor.

At his lowest point, Downey was in and out of rehab, jail, and was a regular fixture in the gossip pages for all the wrong reasons. There were times when his survival seemed in doubt, and a future as a movie star very unlikely.

The actor, however, worked his way back, embracing sobriety and becoming a pillar of reliability. Now, a man who was once hard to employ because insurance companies would not issue a policy on him has become 2015's highest-paid actor on Forbes' annual celebrity list, bringing in $80 million.

How did Downey get here?
The actor worked his way back from career and personal oblivion slowly. He did an arc on TV's Ally McBeal in 2000, after his release from mandated rehab, where he not only won a Golden Globe, he also showed that he could be reliable. That worked to a point, but it was not until 2005 that Downey completely left his drug-fueled past behind and began his comeback in earnest.

From that point forward, he worked steadily, but it was not until his 2008 breakout performance as Tony Stark in Iron Man that he began to ascend the list of highly paid actors. When that movie was released, both its subject matter and its star were considered question marks.

Downey had never carried a blockbuster franchise, and Iron Man was a fairly obscure comic book character. The two, of course, proved a perfect match with the damaged actor being perfectly believable as the damaged technology billionaire-cum-superhero.

That led the actor to his current status as the highest-earning person in his profession.

How big is his box office?
Since mounting his comeback, Downey's box office success outside of the Iron Man suit has been a mixed bag, according to Box Office Mojo. He has scored two solid hits with 2009's Sherlock Holmes ($524 million globally) and its 2011 sequel, Sherlock Holmes: A Game of Shadows ($545 million globally), but he has struggled in other roles.

The Judge was a high-profile bomb in 2014, earning under $100 million worldwide, and 2009's The Soloist made under $40 million (though that was more of a small, art-house film). Downey has had hits as part of an ensemble in Tropic Thunder and Due Date, but nothing approaches the billion-dollar returns he brings in in an Iron Man movie, or as part of The Avengers ensemble.

Ultron

It's an ensemble film, but Downey's Iron Man is front and center in Avengers: Age of Ultron. Source: marvel.com.

Is he worth it?
Downey is one of the few actors who still commands $20 million to topline a movie, and he makes well more than that to put on his superhero outfit. He made $50 million for The Avengers, according to The Hollywood Reporterand almost certainly more for its sequel (though his payday for Avengers: Age of Ultron has not been made public). 

He's expected to make around $40 million to co-star in the next Captain America movie and can pretty much name his price for any Marvel movie going forward.

Given that both Avengers films as well as Iron Man 3 have all topped $1.2 billion in global box office, with Avengers passing $1.5 billion, Downey is worth every penny he gets to play Stark and his superhero alter-ego. He's probably also worth his price to reprise his Holmes role, and he's a reasonable bet in any big-ticket action franchise.

In anything else, though, Downey -- like every modern actor -- is a risk. The Judge had a $50 million budget, according to IMDB, and likely a similar marketing spend, but it failed to connect with audiences. That was not for lack of trying, as the actor made the talk show rounds. The movie did not fail for lack of awareness.

If you assume that hiring Downey costs $20 million up front and a piece of the back end (though the actual price tag can vary based on his interest in the film), the actor is only worth it for certain roles. Downey deserves to be the highest-paid actor as long as he keeps working for Disney in billion-dollar box office Marvel movies. Take him out of that comfort zone, and he may not be worth the price.

Daniel Kline has no position in any stocks mentioned. He is more Hawkeye than Iron Man. The Motley Fool recommends and owns shares of Walt Disney. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.