There are over 10,000 lines of branching dialogue – to give you some perspective, the average screenplay contains about 1000 lines of linear story. Also there is almost an entire novel’s worth of stories and fiction in the game. Most of the ally characters in Dead State have more dialogue than even the biggest characters in Bloodlines. There is a lot of dialogue in Dead State.
There will be dialogue-specific options based on character creation choices.
Many NPCs have special dialogue based on which Allies have been recruited to the shelter.
There are NPCs with existing relationships and if anything happens to one of them, there will be consequences. This may mean they expect you to give preferable treatment to their friends/family or will take drastic action if you were to cause some harm to come to one of them. There are a lot of types of ways allies may have a connection to one another - families, couples, companions that were traveling together before they joined your shelter. There are some allies that become close inside the shelter too.
You can lie or manipulate people based on certain skills. NPCs can lie as well, just not about their stats and skills.
Interaction with survivors depends on the personality of the survivor and how much they approach you. You can be on good terms with people, to the point where they will have your back if you make an unpopular decision.
There are no "romantic" relationships between the player character and other NPCs, though some people in the shelter may get involved with one another.
Having a higher Negotiation or Leadership skill gives more options, but they are not always the best choices, and will sometimes require you to either sacrifice some other asset in the place of another or keep a promise you made.