This is a work in progress but a there are a couple points to note before progressing far into the main story.
*Main Mission 10 N
O SUDDEN MOVES is a point of no return of sorts. In the following mission,
*** Spoiler - click to reveal ***The one with the highest affinity will be part of the group in The Eye, and the one with the second highest affinity will be in The Lodge. Ensure you go to the eye if you want to save the companion you're closest to.
Companion affinity is increased via dialog options with them or in front of them that the favor, making certain decisions in quests and time spent with them in your party. Companion affinity is decreased by saying things and performing actions the respective party finds unfavorable. You will be notified of the character's feeling towards your action by a prompt in the top right corner of the screen stating their feelings.
*Leadership Perk Rank 1 increases companion affinity gain by 25% (although this seems to be unnecessary if you're just looking to get this achievement out of the way).
*Each Companion has a personal quest that will delve into their backstory. This must be completed before any progress is made on their affinity meter. Additionally, they will often request for you to have a conversation with them, furthering their backstory. Ensure you have these conversations as they pop up.
*Romance is of course an option. If that's your prerogative with your companion, then choose "flirt" at every opportunity when speaking to them.
The following is a guide to each character capable of a Maximum relationship level. I'll be updating as the community figures out exactly how to act. Don't worry about this one too much. It's a massive game with ample opportunities to maximize multiple relationships.
Sarah:
*** Spoiler - click to reveal ***1) Always compliment Constellation and agree that the work they're doing is vital.
2) She loves exploration, if there's an option about the vastness of the universe or anything about feeling called toward the stars, this is the one.
3) She is favorable toward showing sympathy toward the innocent. But if there is a scoundrel (like a thief or murderer) she will like if the player chooses to bring them to justice.
4) If you have the ability to avoid a fight through persuasion and diplomacy, she will probably like or love that.
Barrett:
*** Spoiler - click to reveal *** 1) Loves to joke around. If you have the opportunity to give a friendly roast or make light of the situation, this is the correct one.
2) He likes it when you get the job done in a manner that is overall "above board," but he seems not to mind some rule-breaking as much as Sarah does.
3) Dislikes whenever intimidation tactics are utilized in dialog.
4) Initially, he will dislike when you ask about Ervin (his former husband), so wait until he starts to bring him up to you before pressing that sore spot.
Andreja:
*** Spoiler - click to reveal ***1) Unlike the other main companions, she has a tendency to enjoy the more brutish & intimidation dialog options.
2) She will dislike if the player ever second guesses themself or shows any signs of weakness
3) Still has a soft spot and will also favor when kindness is shown toward the innocent.
Sam:
*** Spoiler - click to reveal ***1) He's very practical and grounded in reality, he will often disagree with the other members of constellation, if you have an opportunity to agree with him, be his yes man.
2) Never say anything negative about his daughter, always saying kind things about her will quickly rank up his affinity
3) If a find can be avoided via persuasion, Sam will always approve of this action.
4) As with the other companions, will give positive affinity to kindness to the innocent but will often give positive affinity toward showing mercy even to the enemy.
Please let me know if there is any more information that's required. Wanted to get this out there for everyone to start working toward. Have fun!