I highly recommend you to play through the story by yourselves, without referring to any walkthrough. Here I am posting an achievement guide just in case anyone has difficulties getting all achievements.
There are 10 achievements in total. Five of them are quite hard to miss since you just need to listen through the audio of each level once without playing with the progress bar. Three are achievements of different endings. Only two achievements need special actions and can be obtained using level select after finishing the game once.
When asked 'who are you', choose 'No. 68' then choose the ashtray on the right side.
When asked 'who are you', choose any option other than 'No. 68' or 'detective' (worst ending?)
When asked 'who are you', choose 'No. 68' then choose the pills on the left side.
*Choose 'detective' to see an ending without new achievement.
Listened to Emily humming a snatch of a song from end to end. Obtainable in level 5 (mental hospital); stay in the living room for Emily's whole song. You can use fast-fowarding.
Checked each room in the five maps. Missable rooms include bathroom on the top-right in level 3 (police station) and some small rooms in level 5 (mental hospital). You can use level select to do this; for me I re-entered level 3, entered the bathroom and the achievement popped without the level being solved again.