I’ll preface this by saying “ultimately, play how you want”, but there’s definitely smart ways to play in both Havoc and the “normal” mode, and personally I take the path of least resistance almost always, to benefit my team, and myself.
I have many builds with all classes and have 2k+ hours (40 Havoc, True Survivor) between all of them, almost equally. In the normal game, if I want Grimoires, I’ll specifically choose Zealot with curse removal to help my team. For quick plays, if I’m playing Auric Malestrom and I see specials can become a boss, I’ll take a build more tailored to help with bosses if I see no one else is. If I am playing an Auric with pox hounds, I’ll run something with more aoe to help deal with them. Almost always, I’ll make a decision to benefit my team, unless I’m doing build science lol, but even then I do it intelligently.
I won’t get into the specifics of “how an individual plays Darktide”, but there are options regarding gear/talents, some are just good, some are just bad, some are good with a team, some are bad with a team, some are good while not in coherency, and some are good while in coherency.
As a note, I’ve personally beaten Havoc 40 multiple times and seen it done with every Psyker ability. Smite, BrainBurst, Assail, CritAura, DmgAura, CooldownAura, Shield, VentingShruek, and ScriersGaze. I’ve seen Zealots not use book, and I had an Ogryn carry the team once. There are many viable builds and playstyles, you just need to find/make one and recognize its strengths and weaknesses, and most importantly, play as a team. It’s unnoticeable in the normal game, but you are punished for wandering off far out of coherency 