The quality around here can be intimidating, yep, I get that. The amount of people that can write long, wonderful, plotty fics and I think I can't do that. Then there's the people who write really short pieces, every word cherry picked, and that has me in awe too. But you just have to say, well, I'll do what I do :) Seen that graduation speech Neil Gaiman gave that went viral online? :D

Okay, dialogue. I will attempt to help with this!

1) They don't really talk all that much in the film, so you try starting out with no dailogue at all, just how they communicate non-verbally, with nods, touchs, signals, signs. Perhaps you could then add little snippets that would compliment the non-verbal, like a shout or a well=placed name use. Fics, after all, do not have to have dialogue. It's not a rule :)

2) Crazy time - talk to yourself. This actually does work. I will talk to myself, to mirrors, to walls, to objects. I will stand up and act bits out or try to figure out how characters could move. Not the kick ass stuff, I'm not that bendable ;) But little details. Let the conversation go where it will, backtrack if it doesn't work and try different routes.

3) Rewatch the film :) Yeah, no always possible, but there's Iron Man 2 for some Natasha, that little hint of Clint in Thor, and clips on the internet.

4) Listen to other people. People all have their quirks, accents, dialects, turns of phrase, when it comes to language. Have a listen and think about how your character are going to sound. I'm British, so I want to make sure I avoid slang that wouldn't be used across the pond, for example, and also slang that our Master Assassins might not know if they're not up to speed with pop culture. I say 'shiny' a lot, which crept into my vocab from Firefly without me noticing. I also tend to use 'okay' and 'fun' a lot. What words and phrases would be common for your characters to use? And what's specific to them?

5) And practise really does help :) Just write a lot. You don't have to stick everything online, just practise.

6) Betas. Very useful people :)

Hope that gives you somewhere to start! Go ye forth and have fun :D