Ha!
My Lavellan has totally been naive the entire game. I do imagine her as quite young and feeling at times out of place with all those shems.. She gets swayed easily by the mysterious Elf who seems to know so much and always has the right word and seems so mature and all.. She is good and pure at heart and always gives a second chance to everyone.. etc. When he breaks her heart she just spends the next years being sad and feeling lonely really often..Also because I do all the DLCs with Solas so after Corypheus' defeat there's nothing else to do and I imagine that she just spends the day in Skyhold in sadness dealing with political stuff she detests.. lol.
When Viddasala reveals that Solas is involved she is like "What??? Solas??
How can this be!". And then I headcanon that suddenly she opens her eyes and everything falls in place and she "grows up" in time for the final scene.
My Trevelyan is a total different story.. She's strong, fearless, sarcastic and very self confident, she's part noble so yeah, she knows how to roll and she has fun playing the Game, making a fool of Florianne in front of everyone, etc, and that's why she gets along the most with Leliana's way of thinking.. She knows what she wants and goes after it.. in life and in love (so I pick all the smartass and confident replies with Cullen).
In Trespasser though I imagine her going like "This Council thinks they are going to kick my ass, well they're in for a surprise, stupid Fereldans and Orlesians, I'm not going down" despite what Mother Giselle and Leliana tell her. (In Flemeth's words "As long as the music plays, we dance"). She is angry
shouting at her advisors when hearing about the spy in the Inquisition, etc. Then I decided that during that scene where the mark hits, she has a breakdown and suddenly goes all worried "Oh no my arm, I don't want to die!
". But ofc hubby is there to confort her. So she is quick to shake the bad feelings away and get back on her feet.
She is not evil, just more mature (at least in my head).