O Brothers, where art thou Gamerscore?
Brothers - A Tale of Two Sons is an anomaly when it comes to Achievements. Not a single one will be unlocked through normal gameplay. However, every last one takes only seconds to get and isn't dependent on anything other than traveling a bit off the beaten path.
No, there's nothing strenuous about this list, but most of the Achievements are pretty cryptic in their description. If you're stuck on some or simply want to take all of the mystery out of it, I've put together a guide to unlocking all of them in the order that they appear. I've tried to keep it as spoiler-free as possible, but you probably shouldn't check it out if you want the purest experience possible.
(Update: Note that the "Love Birds" Achievement has two parts. The first action is required in Chapter 1 right before the sheep section. Make sure to read ahead so you don't miss it.)
That being said, 400/400 Gamerscore is right around the corner!
Take a Break
Kids usually play close to home.
As it states in the description, this Achievement will pop outside the brothers' house which is in the Prologue. At the very beginning of the game, as you're carting the father to the doctor, stop immediately before the bridge and instead travel south on some stone steps. Run along the beach until you find a pile of rocks in front of the water. Interact with them as the little brother to begin skipping stones which will unlock the first Achievement.
For your wish to come true, you need something to throw.
At the start of Chapter 1, you can see a well off to the right and a girl playing with her ball in the distance. Go take her ball and throw it down that well, you giant jerkface. I can't believe you emotionally scarred that sweet, little girl for a measly 20 Gamerscore.
It's a long walk.
In Chapter 1, you'll come across a wheel that needs to be run on in order to lower a bridge. Run on it with the little brother, and send the big brother across to pick up a sheep. Carry that sheep back across and to the lower left where the bunnies are playing. Drop him in the coal, which will turn him black and unlock the Achievement.
Are bunnies colorblind?
Also in Chapter 1, at the exact same location as the Black Sheep Achievement, there's a white rabbit hopping among a group of black rabbits. Drop the white one in the coal to knock out this Achievement.
Patience is a virtue.
As soon as you reach the cemetery near the beginning of Chapter 3, there will be a statue off to the right-hand side. Interact with it with the little brother for approximately six seconds. A star will go shooting across the sky, and the Achievement will unlock.
A Sad Tune
Give something back which was believed to be lost.
This one's pretty morbid. At the beginning of Chapter 4, the path forks. Off to the right side, there's a guy who's, well, he's not having a very good time. Climb up the tree with the older brother, and let him down. Then, venture around the right side of the tree and work your way around the ledge. Grab the music box and give it to him. He's probably still bummed out, but you did everything you could.
Find the right tone.
In Chapter 4, after raising the platform that the inventor's stranded on, there's a giant organ-type device in the upper-left part of his home. Turn the crank with the little brother and push the other part with the big brother until you hit the right tone.
A caged heart cannot love.
This is a two-parter. In Chapter 1, right before the wheel that leads to the sheep, there's a bird cage that needs to be opened. Do that, and free the bird. Then, in Chapter 4, immediately before the hang-glider, there's a telescope that's impossible to miss. Look in the telescope, and zoom in on the lower-right. There will be two birds chilling out on a branch, and the Achievement will pop.
Call of the Giants
First take a deep breath.
In Chapter 5, right before using the giant crossbow, go down the path to the right. Waiting there is a giant horn. The little brother is ineffectual, but the big brother has what it takes to make some noise.
Behind the Curtain
On your feet you could never get here.
At the start of Chapter 6, you'll hop in a canoe. At the end of the first straightaway, there'll be a "unique" waterfall off to the right. Go through it to earn this Achievement.
Life, Drop, Slide, Plop!
During Chapter 6, you'll come to an ice cavern which is home to a crying mama turtle. Off to the right are her babies, which have all found themselves in some sort of predicament. Pull one to the top of the giant slide and send him to his mother. There's another waiting to be slid down at the top of the slide. For the final turtle, the big brother will have to boost the little brother up. Once he's down, reunite him with his family.
Rest and smile, sing for a while.
In Chapter 6, immediately after climbing out of the ice cave that houses the family of turtles, you'll come across a two-man saw. Run past it, climb up a ledge, and a bench will be waiting. Take a seat on the bench, and the whales below will put on a show for you.