Most of the characters who share a voice actor have a special pre-battle intro. Of particular note is the interaction between GintokiandJoseph, in which Joseph predicts that Gintoki's next line will be pointing out that they sound alike, or Gintoki predicts Joseph's next line instead.
Gintoki gets another one with Akainu if they're on opposing teams, saying he's an idiot and that his body will break again if he ups his tension anymore, which makes Shinpachi remark that he's thinking of Hasegawa and that he's talking to the wrong opponent. Technically, they arethe same person, though. This is also lampshaded by Kagura if paired with Akainu, outright calling him his "Madao" nickname.
In a non-SJ example, Kagura will say "Urusai!" upon selecting her on the character select screen.
Marth Debuted in "Smash Bros.": The localization of this game brought along with it several characters whose respective series were never officially released outside Japan, such as Luckyman, Ryoutsu, and Saiki. The most egregious case was that of Tarou Yamada — a character from a series so strange and obscure that it didn't even have a page on the Other Wiki.
Name's the Same: Aside from the sole fact that Gokuu's name is literally just "Wukong" said in Japanese on'yomi, Tarou's name is literally just a portion of Momotarou's with the two same characters (太郎).
No Dub for You: The game did not receive any dubs when it was localized and released overseas, likely due to the difficulty of gathering voice actors for all the different series represented in the game.
No Export for You: Averted! After months of silence and despite many Jump crossovers before it not getting the localization treatment, J-Stars received an English release as "J-Stars Victory VS+" in 2015 with added features.
Most, if not all of the voices for the characters are the same as the anime voices (if the character even has an anime incarnation, that is).
Averted with Koro-sensei, who is played by Jun Fukuyama in the anime but Tomokazu Seki in the first animated adaptation (the OVA; barring his VOMIC VA Masaya Onosaka who voices Don Patch). By that proxy, it's due to the anime coming out after J-Stars.
The same also applies to Kusuo Saiki, who while reprised by Shintarou Asanuma from both the flash series and VOMIC adaptations, has Hiroshi Kamiya voicing him in the recent anime series instead.
Also averting this are the Hunter × Hunter characters, but the more clear case is Gon due to Junko Takeuchi to voicing him in the old anime, but is now voiced by Megumi Han instead as of 2011; this pretty much averts Talking to Himself with Naruto.