hi, i'm a long way lover of the movie and always knew about this game, purchased on gog years ago and decided to play it just now.
i'm having a bit of a hard time in understanding what is the point of the investigation, even if i read that this games has randomness going for it, that there are suspects that come by and you gotta understand who's a replicant and who's not.

point is that apart from reading and playbacking pieces of info gotten here and there on my KIA i dont know how any of the above should go. i mean, i am not stuck after the first hour, i'm just a bit confused about how that should play out.

as for the title of the post: i love and have played lots of adventure games since i was a kid, point is that in the menu i can choose the way roy speaks to people OR "ACTIVATE PLAYER'S CHOICE". i did just that, bue i NEVER see any dialog options speaking with people, i also tried a voight kampff on the guy in jail at the police station by mere chance and asked questions beep booping around the interface untile the lady said he was human, but... i don't know am i doing something wrong?

Gathering clues typically means McCoy will be able to come to conclusions, and progress the story.
There are parts in the story where the next action / choice is not available until McCoy has gathered sufficient clues for something (by talking to people, and doing general investigating stuff).

There will also be part in the story where you will be given an choice on how to act or approach a suspect, and having enough clues will help you make an informed decision as opposed to a random one.
Clues are not only about who is a Replicant and who is Human, although that is McCoy's job as a Blade Runner. There are clues for other crimes or people and those fill in the bigger story picture. (There are a few red herring ones too). Make sure that Player's Choice is actually shown as active in KIA.
Not all dialogues will present choices. But quite a few will.
A quick way to test this is the dialogue with Runciter on the first Act when the game starts. Activate Player's Choice and see if you get dialogue options when talking to him.

Wrt the VK; the game's manual offers a general idea on how it works. The guy in jail has already been tested by another Blade Runner so you know what he is supposed to register as in the VK test. He is mostly there for "practice" on a subject and cross check your result.

VK is kind of its own complex thing though. It's best if you get a final positive result for a subject, because that is a good clue usually, but don't sweat it if you fail to get a result.
No VK test in the game is *required* to provide a positive result. You can fail them all, and still finish the game. McCoy can find other clues and there are redundant ones for such cases where he misses or fails to acquire others.

I've written some extended stuff on how it works here -- along with technical stuff too:
https://www.gog.com/forum/blade_runner/voigt_kampff_test

thanks man for your in depth response! actually that diaogue with the first guy was the very thing i tried to use player's choice on, and saw no options. i'll try and tinker around with it.
ill come back to your post whilst playing :)

Keep in mind that you can click on the same person to talk with them further, until you exhaust all points of conversation with them, at that time.

More often that not, clicking on them the first time, McCoy does an "opening" conversation, so there won't be any popup dialogue box to choose a topic. Upon the next click, if McCoy has further stuff to discuss, you get the dialogue selection popup (only in User Choice mood).