It's difficult to give specific advice for different writers, but here is what I do whenever I write dialogue (in fact before I start writing) and I have trouble thinking of speech patterns.
First, I list the characters, then add notes about the type of personality traits I want each of them to have, what they do for a living/education standard, 'social class', and sometimes (when I really have trouble) make notes on how I imagine they view themselves.
Then it's a matter of simply trying to place myself in their shoes and thinking about how my personality would come out in my speech. OR sometimes I base it on actual people I know and think of their intonation and speech patterns when they talk in various situations, for example how do they react to shock? What is their manner when relaxed? If they're interested in someone? Sometimes you end up with dialogue you never expected to write.
Here's an example from a current project:
RINGLEADER (unnamed):
Low-level local street criminal
Brash
Tries to play up to his gang status (i.e. believes he is more of a player than he actually is)
Acts more confident than actually is, especially in front of peers
Disregard for others feelings
Self-obsessed but not to an extreme
Watches too many gangsta films, thinks he's a bigger player than he is
Doesn't like to have his confidence knocked/position challenged in front of peers (becomes embarrassed/frustrated?)
A sample of the dialogue:
EXT. PARK - GANG LOCATION.
HEAD SHOT - RINGLEADER
RINGLEADER
Yeah? Something wrong?
(No response)
CUT TO:
EXT. PARK - GANG LOCATION
Other gang members turn around
RINGLEADER (CONT'D)
I'm f***ing talking to you. Yeah, come here!
(No response)
RINGLEADER (CONT'D)
You f***ing stupid or something? [in Urdu] Iqbal, this f***ing twat is being cheeky!
IQBAL
You go to him when he tells you, yeah?
(No response)
A short pause
RINGLEADER
Seriously! What the f*** do you want? You want to get f***ed up, yeah?
CUT TO: OVER SHOULDER - RINGLEADER
This method is useful for when you revise the screenplay. I always keep the notes handy to refer to when writing, and a quick glance refreshes the memory of what role you want the character to play (always helpful when staring at pages of writing for hours on end).