Most Hollywood stars take their job seriously and do pretty much everything in their power to portray their roles as vividly, consistently and professionally as possible. That is their job, after all. However, every now and then some actor or an actress goes that extra mile to make their character more believable.

Some start preparing mentally, even months ahead, in order to become one with the character and nail down the role properly.

But, what about their physical appearance?

And no, we are not talking about their cutting edge makeup technology, we are talking about pushing their bodies to extreme limits in order to become their on-screen personality and deliver the role as credible as possible.

Here’s the list of 15 such transformations that these amazing actors went through, with their insane weight changes, lifestyle changes and physical training. If we left someone out, feel free to leave a comment.

15. Charlize Theron

Back in 2003 beautiful South African actress and fashion model managed to gain over 30 pounds when she transformed herself into prostitute/serial killer Aileen Wuornos in crime drama “Monster.” One of the craziest transformations ever was completed by terribly looking fake teeth and unflattering makeup.

“The first day we did all of it [hair and makeup] and I looked in the mirror, I was like [gasps], ‘Ok, I’m feeling this – this feels very authentic to me and very real and it’s not a joke.’ I was very happy,” Theron said for ABC News.

Besides her extreme weight change, Theron’s remarkable performance earned her immense critical praise and numerous awards, including the Academy Award for Best Actress and the Golden Globe for Best Actress in a Drama.

The same year she appeared in the heist thriller “The Italian Job.” Remarkable transformation indeed.