How do actors get so lean for certain movies? For example, Matthew McConaughey in Dallas Buyers Club?

Matt McConaughey weight loss for Dallas Buyers club
Matthew McConaughey, on how he lost the weight for “Dallas Buyers Club”:
“Overall, [I’ve] probably got 35 percent less energy, but there’s been plateaus,” McConaughey told the site. “Getting past 170 was really hard, but then once you get [to] 167 the next seven come off easy. Getting past 160, really hard. But then you fly down to 150. Getting past 150 was really hard and then, bam! Got down to 143 and that’s where I want to be. So, once you get past the plateau, your body seems to understand, ‘OK, this is where we’re leaving now, this is where we are’ and so the energy rises.”
So, how did he drop the pounds?
“I’m doing cardio but I’ll tell you what, the more I’ve learned is—and I think it comes with age too—is it’s 90 percent diet,” McConaughey went on. “It’s 90 percent amount and then what you’re eating because right now I’m not losing any more weight if I burn 1,500 calories, two hours of cardio in an afternoon, or if I don’t. It doesn’t matter. It’s a matter of how much I eat or how little I eat.” Full story: Skinny Matt McConaughey Talks Shocking Weight Loss
McConaughey at his leanest in Dallas Buyers Club
This is a video of Penn Jillette, Magician and illusionist of “Penn And Teller” fame, explaining, in plain English, how he lost his weight. I felt this well worth watching -Note that he is telling his story, not selling a weight loss plan.

Performers frequently have to manipulate their weight, both to keep any role they have, and to mold themselves into any roles they are auditioning for – or cast in.  In the case of both McConaughey and Jillette, they both used different means, and each had significant measurable results.
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How do film directors (Hollywood and Bollywood) consult actors (Bollywood and Hollywood) for acting?

Alan Chenkin, Likes to watch films
Casting agents will send directors specific types of actors for certain roles. In addition, the Production/finance companies will recommend actors – many of whom will have a dollar amount of funding connected to them being hired for a film (This is because they have a proven “draw / fanbase” and can fill theaters).  When an actor with a loyal fanbase and a $10 million dollar budget increase is suggested to a director, the director is frequently compelled to cast them in a leading role.  If the Director is a Steven Spielberg, or Woody Allen, they will have the final say in casting, and may pressure the production company to use the talent they want – because they have the star power.
How Casting Directors Work – How stuff works
Directors frequently need to work with the actors to get them in the right “frame of mind” to play their role well, and give them motivation and cues that will convince the audience that they are seeing something real.  Directors will also schedule readings and rehearsals prior to any takes, so that the actors are comfortable in their portrayals. Many times directors will have events like picnics and football games in between shoots to build camaraderie between the actors and crew.
Signed scripts - link to eBay
Signed scripts – link to eBay
Scenes in a movie can be expensive to set up, and anything the director can do (in Bollywood or Hollywood) to get the maximum usable footage is a time and money saving necessity.  In critical scenes, the director may demand many takes, so he has more choices to select the best work.
Your director is motivated to make the best film, and will compel his/her actors to deliver their best.  
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Edited and expanded from my answer in Quora.
The Last Word: