It's a nice game, the story is good enough in my opinion. I also have to agree on the lack of interactivity. All but a few dialogue options have no effect whatsoever. Moreover, finding the clues has no effect on the gameplay, accusing one of the girls in the end will always lead to the same ending (at first I figured that not finding enough clues to accuse one or the other might trigger something else... no such luck...).
I'll hide the next bit as a minor spoiler, select to read on:
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The only actions of real importance are (although other fairly direct choices follow from them): finding the talisman, choosing the right dialogue at dinner with Susan, whether to shoot Kim and who to accuse in the end.
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Anyway, after finding endings 2, 4 and 5 by myself I looked up the way to get endings 1 and 3. For those interested, select the text below, it will give away the endings (big spoilers):
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Ending 1 (Prison): Fuck kim, then say you'll tell Susan anyway
Ending 2 (Accused of murder): Shoot kim without having fucked susan
Ending 3 (Threesome with Susan): Shoot kim after fucking Susan
Ending 4 (Sex with Kim): Accuse Susan
Ending 5 (Killed by Kim): Accuse Kim
How to fuck Susan:
As far as I can tell, if you don't find the talisman at Kim's place you can go back to your office. If you then leave the office you'll be invited to the restaurant by Susan. If you pick the 'nice' answers she'll offer to go to her place.
How to fuck Kim:
Find the talisman, then tell her you're going to tell susan about it. She will offer to do anything you want. Keep your secrets to yourself if you don't want ending 1

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End of spoilers