You’ve probably heard that if you react or behave in a particular way in an improvised scene, you should continue to behave or react in that way throughout the scene. That’s good advice. It’s no fun to see a character react to something in a specific way and then drop it for the rest of the scene. If your scene partner does something and you react suspiciously, you should probably react to other things suspiciously too. That’s how you create a game1 for yourself.
The key though is to let yourself be provoked into these reactions by your scene partner. You shouldn’t have to invent things in the scene to treat suspiciously. It’s far better to be provoked by your scene partners actions instead. You could notice the bookcase and the copy of the Bible, pull it out and glower suspiciously at it. But it would be far more interesting if your scene partner said, I want to show you something and pulled out a book from the bookcase and then you became suspicious.
In rare cases, your scene partner may not be doing anything at all and then you have to actually discover things on your own to provoke you. But most of the time, once you have a template for how this character reacts, you should focus your attention on your scene partner and when your gut tells you “React!” go for it.