In Baldur's Gate 3, during Act 3 at the Circus of the Last Days, you will encounter Zethino, a dryad who will test your knowledge of your chosen love interest. Zethino will ask you three questions about your partner's deepest joy, desire, regret, and fear. However, the questions and answers will vary depending on the character you've chosen to romance.
For Astarion, the correct responses to Zethino's questions are as follows: his deepest joy is “When he's elbow deep in gore,” his main desire is “Revenge,” and his biggest regret is “Breaking a nail.”
Gale's correct answers are: his deepest joy is a shiny red apple, he desires to be on his balcony in Waterdeep, and his regret is that he thinks he, and the world, might be better off if he were dead.
Halsin's correct responses are: comfort doesn't come naturally to him, as he is restless and roaming, his main desire is to protect his grove from threats, and his regret is allowing the shadow curse to blight nature for a hundred years.
Karlach's correct answers include: her deepest joy is Gortash, her desire is bashing baddies interspersed with victory sex, and her regret would be settling down in a quiet village with a nice partner and kids.
Lae'zel's correct responses are: her deepest joy is to find the truth about Orpheus amidst her queen's lies, her desire is the night she spent with the player character, and her biggest achievement would be being known as a liberator of the githyanki people.
Minthara's correct answers are: her deepest joy is picking off her siblings one by one, her desire is the player character, and her biggest regret is allowing herself to be captured by the cult of the Absolute.
Shadowheart's correct responses include: her deepest joy is good company and an even better vintage, her desire is to be discreet, and her biggest fear is that she can't swim.
Wyll's correct answers are: his deepest joy is the memories of the first night getting to know each other, his desire is his father, Ulder Ravengard, and his biggest regret is leaving behind his father and his city.
It is uncertain if characters like Minsc or Jaheira can participate in Zethino's love test, as they are non-romanceable companions. However, the guide will be updated if new information emerges.
Source: Baldur's Gate 3 community discussions and forums.