It's no great secret that in every game we've had in the past from bethesda game studios the AI has been at best completely terrible. From 'must have been the wind' to 'blah blah blah, bleed to death, you're [censored], I'm great, what's this; I'm dead?' and we've had characters who just... don't feel like people. as if they've somehow decided upon the most perfectly worst place in the uncanny valley to sit on.
That thing with the companions. Y'know where the player tells the dog to get a wrench and the dog does that? Have something like that happen to companions. You yourself can't hack a computer? Get your friend to do it. Can't unlock/lock/knock down a door? Have your friend do it. Can't suave your way past a person? Have your friend do it. Ideally, companions should often be able to make up for the player's weaknesses. For more added fun, have these characters sometimes do [censored] that you didn't tell them to... It could be something useful like taking a hostage when you need one, or pulling you out the way of a tripmine the player didn't see... or it could be something detrimental; like stealing something valuable and putting it in your bag, or killing a character you're talking to, or insulting a tribal chief while you're completely at his mercy. Edit: This really depends on what companion you get though, and how happy they are with you
Ideally, characters should be intellegent enough that they don't need to be invulnerable to not die stupidly. The first step of combat is to not have characters charge forward like idiots when they see an enemy. A melee character who sees the player with a gun should, if not confident in their ability to smack the player before being shot, either look for a different path to get to the player, or run and hide for an ambush. Ambush ambush ambush. (though if intellegence is really low, and drugs are really high, maybe charging forth into bullet is the best way to go) Clogging the player around the head when they pass through doors for example, or dropping a grenade and running out of a room are good examples of this. (just never force a head clogging on the player if the player/character is wise enough to spot one.
Ideally, an npc should actualy be able to gage the threat of the enemy. one man with a stick? call to his friends and attack. A pair of power armoured, plasma wielding death machines? Run to friends, consider organising attack, possibly run away.
What should be fun, is if we had panic (you know, how in arkham those with guns would become erratic, move around a lot, fire at places they think batman could be) Outright running for their lives, and surrender. Not the silly skyrim surrender, an actual throw down of the weapon and submisiveness (though they could try to kill you when you're not looking)
But we also need more civilian stuff. They need to move around like real people. slouch on chairs and lean on tables. Play pool, cards, mutant cockfights. Consume jet. Drink as they move around the wasteland. Find a small corner to huddle in when they find themselves in the ruins of the old world. Hide underneath beds and tables, Knock over beds and tables for cover. Even things like knocking on someone's door and having that person open the door for them. But it's the small things that matter too. A wastelander is going to walk differently than a socialite. The way he moves will be different. Similarly a military veteran is going to hold his gun differently than a noob who's never carried one or an action movie fan, and a drunk is going to drink differently compared to when he was sober.
TL;DR
Add some human to human characters.
Also, dogs, and all variations of either of these creatures.