It depends where and when i meet this person and how she/he looks like.
Lets use a real example here.
My sisters boyfriend is really neat, well dressed and reserved.
He is at our place pretty often and yeah, he knows us pretty well by now.
When i first met him, i knew exactly that he is one of those really dry motherfuckers that doesnt have any sense of humor and doesnt talk much at all. You know what? I was right and i fucking hate that guy.
Wish he'd open up more.

On the other hand, i rarely judge people from looks.
But its pretty easy with all stereotypes around here.

When I first saw this girl in my class, I thought she was nosy and boring.
We're now like sisters xD
When I first sat next to this weird guy in my class two years ago, I thought he was retarded.
Now we're together.

Ohh the irony.

From the scientific side you judge people by their smell on the first sight. Subconsciously your brain decides whether the smell of a person appeals to you, meaning if the other person fits to you. Only after this first judgement you go by other criteria.
This also why social judgement is harder on the internet.

I probably only notice people who smell of bacon, metal, roast chicken and orange juice, then. I love those smells!

I was best friends with this girl in my year for 3 years. The first time you meet her, she's lovely. She's pretty, warm-hearted, kind, all that shit.
Now I can't stand the fucking sight of her.
She's a backstabbing, manipulative, little bitch. (I know I sound like a jealous high school girl but really, its true.)
Some people just aren't who they seem to be.

I once hated someone by how they acted but then a couple weeks later I came up to him and we are best friends now

Good question,usually i don't judge people by how them act or try to act just when i meet them for real.

I typically judge by the facial expressions and body language they give off. If they speak around me I can sometimes judge how they are based upon the cadence of their voice. But I find most judgement-casting unfair so I try not to be too immediate with my opinions on a person before I cactually have an in-depth discussion with them. Sometimes people surprise me, since typically I am pessimistic in my judgement sometimes.

I judge people by getting to know them properly.
For strangers, I look at their appearance, characteristic but mainly on behavior so I can judge them with understandings.
But yeah I do this occasionally when something catches my eyes.