In an interview with Teen Vogue Sam Clafin confirmed that he would be carrying Mags in the movie.


TV: Do you like your blonde hair?
SC: It’s definitely taking some getting used to. I have a tan and a six-pack now after four months of hard work. I feel slowly but surely like I am Finnick Odair.
TV: Your training sounds intense!
SC: From the moment I set foot in Atlanta, they had me at the gym, working out twice a day. And then stunt training, it’s nonstop. To be fair, I spend a lot of the film carrying an old lady on my back, as Finnick carries Mags for the majority of the arena scenes. I feel like every day of filming is a workout, running around Hawaii with little old Lynn Cohen on my back. As for my diet, I think I’ve eaten more chickens than there are in the world. [Laughs] I would eat chicken and asparagus for lunch and dinner, and for breakfast I’d have an omelet and oatmeal. And I had that every day for four months. I’ve talked to my friends and said, “Oh my God, I’m so depressed, I just want a burger.” And they’re like, “You don’t know how lucky you are to be doing what you are doing.”
Source: Teen Vogue via. mockingjay.net