As you probably already read in the question on dialects, Yakuzas are often pictured speaking Hiroshima-ben on TV. According to Japanese friends, this has probably as much to do with the fact that Hiroshima-ben naturally sounds quite hard to the ear (whereas soft-spoken Kyoto-ben is the typical dialect choice for cute, feminine characters) as any real-world trend.
In reality, I'd say your average yakuza speaks a slang-heavy, ultra-masculine version of whatever his (or his clan's) native dialect is. As it happens, many yakuzas do come from Hiroshima and the West of Japan in general, so Kansai-ish accent is usually a safe bet (once again: Kansai-ben(s) tend to sound harsher and more direct than other dialects, so there's that too). Pretty sure the none-to-rare Osaka yakusa speaks exclusively osaka-ben...
As you already guessed, among obvious (possibly stereotypical) characteristics are:
1) extreme rolling of 'R's and ample exaggerated consonant gemination...
2) liberal use of profanities (このやろう、おんどれ etc.) and strong interjections (ほっっら etc.)
3) casual use of boastful/demeaning pronouns (われ、俺、きさま etc.)
But frankly, you will hear all of the above (and more) by talking to any sufficiently cocky working-class Osaka resident.
My nifty bilingual kansai-ben phrasebook has a (slightly tongue-in-cheek) "angry speech" section. Here are a couple of the more straightforward ones:
ワレ、どつきまわすど！ → "I'll beat the crap out of you"
血い見るどわれぇ【ちいみるどわれぇ】→ "So you wanna see some blood?!"
クソガキがいちびりやがって！しまいにはイてまうぞ！→ "You're asking for trouble, kid!"
おんどれ、ええ根性しとるやないけ！ → "You are starting to piss me off"
One more for the road?
ワレ、目ん玉ほじぐりかえすど！ → "I'll rip your eyes out!"
Once again same warning applies re. real yakuza vs. average Kishiwada nomiya dweller (never faced a pissed-off yakuza, so couldn't tell you)
PS: all apologies if the above is more an overview of regular rude/harsh Kansai-ben than bona-fide Yakuza-speech, but in my limited (courteous, non-confrontational and definitely non-violent) interactions with these types, that's pretty much what they've always sounded like.
Addendum: talked some more with (Japanese) friends about this and the consensus was that there ultimately isn't any fundamental difference between "Yakusa-speech" and heavily-masculinised working-class osaka-ben (or hiroshima-ben, or whatever you pick).
There are, however, a few words that are exclusively Yakuza slang, e.g.:
チャカ → gun (instead of 鉄砲【てっぽう】)
サバく → to sell (instead of 売る)
シャブ → drugs (methamphetamine)
Not to mention common terms for Yakuza hierarchy (チンピラ, おやぶん etc.) and derogative nicknames for the police: 犬【いぬ】, ポリこう etc.
Good luck placing those in everyday conversation ;-)
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