Its not hard to get into someones show, and whether you agree or disagree, that's up to you. but again, this is where defining "difficulty". anyone can do it. can they do it instantly? no...and i'm not saying that, but the process of learning a new persona is easier when you enjoy meeting contrasting views.
So i will say it again, its not "difficult", it does take time to fully understand, but if you rush and if you don't enjoy learning someone who could potentially be your exact opposite, its not really deep.
I'm not talking about knowing someone to the point of potentially reading their minds, or knowing everything they do or say word for word.
i'm just talking about generally knowing the type of reaction and type of things that they say and getting knowing their personality and understanding it to a degree. and putting yourself in someone shoes (which if you look up in google, directly means putting yourself in someone's situation, not mind).
Figure out the way they talk and what they are saying, you hang around enough, you start to get them. its not like you're analyzing a crazy person (but even then, you can learn something from one), and its not like you need to know someone so deeply to write about a character based off them. Otherwise, you'll constantly need to find real people just to write about them.
this is me, being on topic.
Dialogue is a completely different thing. it is all about portraying what you know and often times you can get lost in what you know, or you present it too direct it feels bland, rather than providing it subtly.
For me, i usually develop the character to the point that they feel real enough for me. And make them watch me write over my shoulder. Often times, i get it right with whot hey are, but sometimes its hard to convey what their saying to the reader, so i end up having to "compromise" with my characters.