Let's see if this is a useful starting point for anybody. It's purposefully vague because 1) it's more readable that way, 2) I'm not obsessive about it and 3) FFG would probably not like for every detail of their game to be spelled out to the public.

Mood : Depending on his mood, he can adjust the cards he plays to shift 1 less or 1 more (in addition to matching Plots). This can allow you to chain several cards together, continually playing either Light or Dark cards, or quickly shift in order to play a high-cost card in the other direction.

NOTE: More than half of Louis' Dark cards allow him to draw Dark cards in return. He's a vengeful sumbitch.

On the Take: Louis deals with his ties to mob boss Mr. Li.
Depending on whether he chooses to talk peacably with Li, or try and take him out, he becomes harder to trap in a Fight, or limits his choices in a Fight.
If he talks with Li, he either successfully negotiates his way out of the mob (+7), or is coldly refused (-1).
If he fights Li, he either shoots him dead (+4) and loses his ability to get Li favors, or Li escapes his grasp (-5).
LIGHT: Using his mob ties for all their worth, obtaining favors, manipulating evidence, and finally whacking that jerkass Tanaka.
DARK: A few fights, but mostly attempts to counteract Louis' progress towards the murder case.

Sara: Louis attempts to reconcile with his estranged wife.
He can choose to try keeping her from walking out on him (allowing him to spend time with her to get favors), or give the trial separation a go (letting him focus more on following leads quickly).
If he convinces her to stay, he has to spend quality time with her and avoid being caught up in mob affairs. he either gets her back and happily conceives with her (+7), or she serves him "the papers" (-1).
If he lets her have space, he has to keep his police business away from her district. She then decides to come back to him (+3), or gets killed by the murderer, causing Louis to swear revenge (-9, but only -2 if Louis bags his Guilty Hunch).
LIGHT: Noticeably, all are free to play. They help Louis get Sara favors, and help him avoid trauma from fights, and having Dark cards played on him.
DARK: Lots of cancellations, and fights, designed to worsen Sara's impression of him.

The Krausey Case: Louis tries to finally resolve a 20-year-old murder case involving a well-liked socialite and an innocent little girl.
If he focuses his policework on this case (instead of the current murder), he can catch a break. Otherwise, he turns up little.
Once back on the trail, if he resumes following leads, he'll find the man and be inspired (+7, future plots start 1 good), or finds him mere days after his death (-1).
If he gets frustrated, he won't get any help from the NAPD, and either determine on his own through thorough research that the murders are linked (+1, +5 to Guilty Hunch), or just shoots the old man in a dark alley (-5, future plots start 1 bad).
LIGHT: He can get some quick favors, warrants, manipulate evidence considerably, and plan his future Light draws.
DARK: Gets Louis into fights, delays him, moves him about, and causes him Trauma.

Interesting! If you get no objection (I think you shouldn't, you give the plot values, but not how to affect them specifically, you give general themes of twilight cards, but not specific texts), that'd be a great short series to read.