Have characters' voice actors in Fate/ always been the same? I've been wondering, between Fate/Zero, Fate/Stay Night Realta Nua, Fate/Stay Night (anime), Unlimited Blade Works (movie), Fate/Extra, Fate/Extra CCC and Fate/kaleid liner PRISMA☆ILLYA (and to an extent Carnival Phantasm), have the same Japanese voice actors been employed for returning characters (e.g. Saber-Arturia, Gilgamesh, Rin, Ilya, EMIYA)?

Also, if they have remained the same, is there any confirmation that Fate/Hollow Ataraxia on the PS Vita will follow the same trend?
This will only be a partial answer addressing some of the major characters, because there's too many characters in the Fate universe for me to be bothered tracking all of them down. Broadly, though, with the exception of male characters like Shirou and Kiritsugu who are played by women when children and by men when adults, I know of only one case in which different VAs have played the same character in different entries in the Fate-universe. 

This one case is that of Sapphire in Prisma Illya. The unfortunate passing of Matsuki Miyu after the airing of 2wei Herz led to her being replaced by Kakazu Yumi in 3rei. 



Re: Fate/stay night - the casts of the 2006 anime, the UBW movie, Realta Nua, and the 2014 anime are all the same, at least for the major characters, so I haven't specifically made a note of that in the list below.


Saber (Arturia)


voiced by Kawasumi Ayako in Fate/Zero, Fate/stay night, Prisma Illya, Carnival Phantasm
does not appear in Fate/Extra, Fate/Extra CCC, Carnival Phantasm

Gilgamesh


voiced by Tomokazu Seki in Fate/Zero, Fate/stay night, Fate/Extra CCC
in Prisma Illya, his child form is voiced by Endo Aya, but the guttural yelling of his black form in the last few episodes of 2wei Herz are also by Tomokazu Seki
does not appear in Fate/Extra

Rin


voiced by Kana Ueda in Fate/Zero, Fate/stay night, Fate/Extra, Fate/Extra CCC, Prisma Illya, Carnival Phantasm

Illya


voiced by Kadowaki Mai in Fate/Zero, Fate/stay night, Prisma Illya, Carnival Phantasm
does not appear in Fate/Extra, Fate/Extra CCC

Archer (EMIYA)


voiced by Suwabe Junichi in Fate/stay night, Fate/Extra, Fate/Extra CCC, Carnival Phantasm
does not appear in Fate/Zero, Prisma Illya



This Crunchyroll article lists some of the voice actors for the Vita port of F/HA; all the ones that are listed are the same people that played those characters in earlier Fate entries. The Japanese Wikipedia article for F/HA also lists some more VAs for F/HA, citing Type-Moon ACE Vol. 9; all of these match up correctly too. 

Besides, I don't think it would be in Type-Moon's best interest to change VAs for any of the major characters - imagine what a fit fans would throw if their waifu/husbando got a new VA!



You didn't ask about Tsukihime, but do note that the voice actors for Tsukihime in the 2003 anime all differ from their voice actors in Carnival Phantasm. If the Tsukihime remake ever happens, I suspect that they'll stick with the Carnival Phantasm VAs rather than the 2003 anime VAs.