Though this 2da definition of RPG is slick, elegant and hard to counter, I would like to go back to my definition of a RPG and put it to the test with the JA2 and similar games like Fallout Tactics or Silent Storm series.
1) Player character as indirect medium between the player and gameworld.
Here the main difference between classical and tactical RPGs lies. Classical RPGs revolve around single player character: that is, you can have a party or allies, but the "main unit" of the game, the main protagonist is the player character. Tactical RPGs have squad as the "main unit" with each member being more or less equally important, soloing notwithstanding.
2) Outcome depends more on character skills and less on player' skill. Player chooses, character does.
Absolutely true. Player makes decisions, characters make them happen. Skills are indispensable as ever.
3) There should be a lot of such interactions, obviously.
Once again, absolutely true. Aside of combat, it's lockpicking, traps, barter, exploration and similar interactions.
4) And choice, after all. Otherwise, it would be "Predetermined character playing game".
You can create your character or you can compose your squad to your liking; considering that the "main unit" of such games is squad you can see squad being a sum of its parts, analogous to a single character. Squad of three machinegunners, two grenadiers and sniper in Silent Storm is analogues to a pure combat single character in classic RPG; add medic and engineer and you get more hybrid squad - akin to leveling up medical and engineering skill for a single character. All of this effectively affects your gameplay.
A question arises: where is the difference between tactical squad-based RPGs and RTS, brother? RTS games do not have any "main unit" at all: all the units that you build/hire are just tools to achieve your goal. Player has little control over the given units: no choice of skills, stats, equipment; power progression of the game is pronounced not in the skills/equipment but in unit themselves. There no need to build an army of Trikes when you can build a horde of Devastators in Dune 2, zerg rushes notwithstanding. Usually it's several types of predefined and absolutely similar units, replaced and copied without a thought from the player. RTT games come closer to a RPG: you can control equipment, there is a skill progression and each unit has more personality and unique qualities. Take Ultimate General Civil War as a good example of such game.
There is a clearly defined border between genres (being a metalhead I know a lot about genre-faggotry, kek):
CRPG (Fallout, Arcanum, Age of Decadence) - Tactical RPG (JA2, Silent Storm) ---- the border lies here ---- RTT (UGCW, Men of War) - RTS (C&C, Warcraft, Dune)