In order to influence the role of a character, the game must allow decision dynamics that can craft a flowing evolution of choices. The dialogue also must be meaningful, both to the player, and to the characters in the game who receive it.

In Witcher 2, decisions come down to hard For versus hard Against, Black vs White, path 1 or path 2, allowing no reflection of detailed consideration of events. Also, the events decided upon are often regarding opinions of distant matters, and not the player making directly-pertinent determinations that reflect player character.

Also, the dialogue is dull in general, and most of it is to trigger action-based quest progression, without the dialogue itself being important. Lots of the dialogue can be skipped through without the game experience being affected much by it - because the dialogue is often irrelevant, tangential, hard-polar, and placed there to trigger quest progression rather than unique quests emerging through thoughtful dialogue and layered decisions.

On top of being boolean, and unrelatable in its choices hard-polarity, The dialogue expressions seldom represent well the choices that the player makes, and the outcome is random.

Also, the quests are basic in design, generally one-step action-based completions, and then return to sender. The game pretenses otherwise by having some quests linger through acts before they can actually resolve, but they contain no tasks giving direct involvement along the way, so they just sit there until completing the other independently-assigned quests amount to those parent themes also closing.

All these things mean that the Role-Playing in Witcher 2 is not more than any of a great slew of Action-Adventure games, and is even less than some. It certainly doesn't quality Witcher 2 for a classification of RPG - a person can also imaginatively Role-Play in a FPS, if they choose, but it's the details which a game supplies on its own and not those a player projects onto it that determine its classification as RPG.

Witcher 2 was branded "RPG" for a lowest-common-denominator-gameplay conditioned console generation, but it is not an RPG, then or now.

"What are the prerequisites of the games so that they could be called RPG games?"

You have a lot of good arguments here, where I'm gonna point out several of them

- the dialogue is dull in general, and most of it is to trigger action-based quest progression, without the dialogue itself being important.
- the quest is basic in design, generally one-step action-based completions, and then return to sender.
- The game pretenses otherwise by having some quests linger through acts before they can actually resolve, but they contain no tasks giving direct involvement along the way,

To be fair, most of (in fact almost all ) RPG games have these, especially Bioware games. And yet, people keep comparing rpg games to their company.

MMORPG - which stands for massively multiplayer online ROLE-PLAYING GAMES have these specific kind of things. For less-notorious example (other than WoW), Ragnarok Online 2, Cabal Online have none memorable quest that I can ever remembered playing it. That's because the storyline is so dull and boring, that people are playing it just for grinding levels and PvP with each other. You said " Lots of the dialogue can be skipped through without the game experience being affected much by it " . Familiar tapping the skip button on the dialogs if you play mmorpg, because you simply doesn't care?

Now why did I mention about mmorpg? This is the irony that I want to point out to you. If the prerequisite for Role-Playing games like you have mentioned above is :
- "the game must allow decision dynamics that can craft a flowing evolution of choices. The dialogue also must be meaningful, both to the player, and to the characters in the game who receive it. "
If then, most of the mmorpg games ( Ragnarok Online 2, Cabal, Aura Kingdom etc) should NOT be called ''MMORPG''. They should be called "MMO-Action Adventure" instead.

Back to the RPG genre, like I said above other games also have similar issues. Whether the dialogues and storyline are boring or not, it is entirely subjective. At least, The Witcher actually give the real consequences of your choices, where you literally can determine the fate of a kingdom, and the course of the story. Most other RPG games are straight linear in terms of storyline, and didn't let you choose your own story.