There are two fundamentally wrong assumptions here.
1. Good acting is about telling the truth, the truest truth you possibly can.
2. Character is not an outside thing you put on like a costume, it must arise from within. The character is never someone else, it always you.
Human experience is 100% subjective. It's not like we can just shrug off the "my perspective suit" and put on another. Your personality, belief system and cognitive links are so complex and deep and so much a part of you that you can't switch them on and off. Even if you were pretending to be someone else and no-one noticed how disconnected you were, you could only portray them as you see them, which isn't them at all is it? Two dimensional characters and broad stereotyping are often a result of this "put on the character" kind of thinking. It's surface level, judgemental and never believable.
OK, but what about those actors who totally transform? They're not being themselves. They're completely different from character to character! They walk differently, talk differently, their personality is different. How can I say they're not pretending to be someone else? Are these not affected behaviours?
This is where imaginary circumstances come in to play.
Stanislavski talked about the Magic If. Meisner said that acting is living truthfully under imaginary circumstances. Herein lies the secret. This is how transformation is possible. You can't fake it and it can't be applied from the outside, it must always come from within. You only ever have yourself to work with, but your imagination is infinite.
I started acting when I was about 6 or 7. At the time, all I wanted was to be someone else. I was bullied by my peers, I hated the way I looked and I treasured the chance to be someone else one afternoon a week, checking my reality at the door. That's what got me in, but that's not why I stayed.
Spend any reasonable amount of time studying the art of acting and you realize that it's about facing your demons, not running away from them. By the time I was a teenager, my reasons had shifted. I wasn't happy with my life, but I wanted to change it, not escape it. The rehearsal room was the only safe place to be myself. To delve deep into my subconscious, to learn, to grow, to work out some of my angst. To be free.
Acting isn't therapy. Your acting coach does not (typically) have a degree in psychology and you should never confuse the two. It does, however, require that you have an intimate knowledge of yourself.
As an adult, I love theatre more than ever and the magic is certainly still a part of it, but my reasons have shifted once again. Acting is no longer about escaping reality and my demons and I are great friends at this point. The Ancient Greeks used theatre for Catharsis. For me it's about Empathy. Whether you're attending a powerful performance as an audience member, taking a beginners acting class for the fun of it or working as a professional actor, you may not be thinking about it, but you're strengthening your empathy muscle. You're opening yourself to experience someone else's reality. By experiencing the truth (especially someone else's truth) under imaginary circumstances, you're learning how to understand, if not to love, your neighbour. You're building your humanity. And the world needs more of that I think.
There's not a lot of truly powerful theatre out there at the moment, but it is out there. Go see theatre. Take an acting class. Take a giant step outside your mind and watch how it changes your world.