How do you decide your party composition ?

I'd take Varric or Sebastian for the Varterral fight - archers are good against large bosses.

For Varnell, Gravitic Ring is a must if you're a Force Mage. That's a tricky fight though!

- What type of quest it is. If I can get interesting dialogue from the characters or not. Like I will take Anders and Fenris on Merrill's final quest just to hear them lay into her for being stupid.

- I will usually take the sibling as much as possible. Aveline is usually a staple in my party because I usually play rogue or mage in this game. I also tend to take the romance often as well. Merrill and Izzy for bro Hawke and Fenris for female Hawke.

- Characters who are helpful like Anders if I'm not a mage.

- But above all, I tend to take characters I like so Fenris, Aveline, and one of the siblings are my usual pick.

I normally go solo sarcastic hawke

I tend to think "Who would it make sense for Hawke to call on for this particular job, and who would actually want to participate?", and then go from there. Usually a warrior, two rogues and a mage. I'm playing my first bow rogue on Nightmare atm, and my tactics tend to shift back and forth as I change party members.

Fenris is a beast to unleash on a mob, but an enemy tank will slow him down and his wild swings make it dangerous to have other party members running around in melee with him. Better to let him work out his frustrations and pick reinforcements off from a distance with Varric and Bethany or whoever to keep him from being overwhelmed. That's my current outfit. Aveline is a comparable tank, and safer to surround with melee support, but her principles could get in the way on some of the more clandestine jobs. And besides, she has a full-time job of her own that demands her attention, and can't come running for every little thing.

All the mages work for elemental weapon buffs and healing, and each have their special quirks. Bethany is a go-to as your sister, but not for jobs that expose her too much fighting in the street in broad daylight. Anders is the most experienced and extensively trained mage and occultist, but his personal issues can also make him a liability. Merril is adorable and nostalgic, and also a pretty respectable nuker. But at the same time, she has the weakest stomach of the team and isn't much use in social situations with anyone but other elves.

Isabela is both lethal and surprisingly study, but she´s too erratic and unpredictable to have mages launching AoEs from the back without risking her getting fried while she's dodging and weaving between enemies. And besides, I can't imagine her being enthusiastic about every boring grunt job I volunteer for. Meanwhile, Varric is a solid damage dealer, friend, confidante, entertainer and negotiator. I rarely ever go anywhere without him.

Take from that what you will. The game doesn't punish you for mixing and matching every few quests and putting together a team that makes sense for you RP-wise, as long as you adjust your tactics sensibly so they don't directly sabotage each other. At least on Nightmare.

At the end of an act or during a final battle, I usually bring Varric, the sibling and the LI. Its a best friend/family/lover thing.

If the sibling isn't available, then Aveline because she's been there since Day 1.

- Aveline has a perma-spot in my party.
Best tank I have EVER seen
Has been there since the beginning

- Varric gets the second perma-spot.
Excellent archer-rogue
Bianca tree
Relevant, story-enriching dialogue

- Anders or Fenris
Extra power or tanking
Either having extra healing or another frontliner.

- Myself
Mage (some Creation, Arcane, Elemental, dip in Spirit [Spirit Bolt] and Primal [Chain Lightning])
Spirit Healer
Force Mage

I've said this in relevant threads; no one of us is filling only a single role, so we can adapt as needed.

When they weren't busy killing each other with friendly fire and actually killed other stuff, I noticed Fenris was the bigger damage dealer out of the two, so I opted to swap Carver out for Anders for Malvernis. But it didn't really matter in the end. Malvernis got stuck behind the stairs/ramp when he turned into dragon form and I safely finished him off at range while Fenris stood there, as the dragon was stuck in a location he couldn't reach, haha. A few demons popped out but it was ridiculously easy,thanks his big dragon ass getting stuck. Phew I got lucky with that fight!

Now I'm about to head towards Cory so I suppose I'll swap Carver back in and leave Fenris behind since they keep hitting each other with their swords. I wasn't in the mood for Anders and his meltdown along the way, which was initially why I wasn't planning on bringing him. but from a tactical point of view it's the smarter choice.

But as far as my usual thing, all across the DA games, I generally prefer to run with 1 warrior, 1 rogue, 2 mages.